[Redmi 9A Questions & Answers] Ivan.Adriazola:
Hey I faced similar or even worse (twice), I had a similar story to yours,

FIrst itme:

I first bricked my phone, but it was cool, the phone just flashed MIUI for a fraction of a second when trying to go to recovery, other than that, nothing, I tried flashing, wich made it much much worse, black screen, no key combo did nothing, windows not just did not recognize it,it told me there was no way of recognizing it, (error 43 in devices admin, its not posible to reinstall the driver, nor do crap at this point) filter wizard showed nothing, bypassing imposible, etc.

It got pretty similar to what you had:

I just left the phone do some charging discharging cycles (at least 4 days with me checking if the phone did anything at all almost once every day, just so you know the phone was really bricked) and it eventually responded after all that (Im not quite sure what fixed this, I dont kno if it was complete discharge or what, but something happened and it vibrated when holding the pwr button.

Once that worked, I managed to install the filter by pressing volup voldown and conecting the phone to the USB port (dont use 3.0, it works worse for some reason) at the same time, you need to connect and press the buttons and connect at the same time (

at 1:30 he does it, video on spanish) and then you can install the filter.

Then you can Bypass and attempt to flash with SPflash tool, I recomend autoconcect, UART never made it for me, Im thinking thats for when you can put it in EDL mode by presing the test points, but I dont know enough to say for sure, point is, UART never worked for me, even tho I eve know what comms I was using.

second time:

I was running Custom ROM ArrowOS 12, ArrowOS13 came out some weeks ago, so I decided to lash it, it was better in some areas and worse in others, the contacts app did not work, but now Volte fingerprint(irrelevant to us redmi 9a users) and face unlock did so I decide to test some stuff, I wasnted to flsh boot.img from original MIUI ROM on top of ArrowOS13, to check if that fixed some stuff (that worked wonders on ArrowOS12) that bootloped thephone but it was fixable from recovery, I did that and decided to flash MIUI ROM to check if flashing ArrowOS13 from MIUI ROM worked, and flashing MIUI bricked my phone.

unrecgnized with error 43, not posible to fix from software, black screen no response whatsoever from any key combo (all 7 combos) no charging, no filter, no nothing, I know I had around 50% battery (days ago I remembered the exact number, now I forgot it, but just so you know, its not a hunch, I knew it had battery) I tried doing the same stuppid crap, charging and leaving it alone for couple hours, but after one night nothing worked, so I assumed I would have to repeat the procees I did before (I wrote it complete on an unbricking thread so I knew I could go back there and redo that or variate it until it worked), but I decided to do one final thing.

I knew it had battery, so I decided to try and forcedrain the battery, I took a piece of TP and gave it a turn around the phone over the buttons (this is just to not dirty the phone) then with electrical tape, I left the power button and the vol down button pressed) this for around 7 hours my plan was leaving it like that for a full day, but around 6 hours I thought I heard a vibration from the phone, so I kept it with me, then after a while I thought it happened again, but I also had my own phone with me so I thought it was that and some other things distracted me so Im guessing I missed a few, but after that I started to take time from every vibration and yeah, around every 8 - 10 minutes the phone vibrated

I decided to connect the phone (I hadnt before because it would just slowdown the battery draining process) and the phone conected to the pc, still bricked, but it was a lot better, I quickly installed the filter, wich I dont quite remeber, but it wasnt easy, I had some hickups, cmd bluescreened my PC when using Bypass like 6 times, so I switched to powershell and that worked (to do that you need to press shift+right click and open powershell here, then run "python main.py") that worked, but I did not flash using SP flash tool.

I was trying to avoid turning on the phone for fear it would go back to the previus state, but after a while pluged on the PC it showed the no battery logo so I tried to tun it on and it was bootlooped, I was able to go to recovery and fastboot (not normally, I pressed the keys, the phone try to go into those menus, it showed no battery, and in the next cycle after splash logo no matter what you did it sent you to the previus key combo menu) I tried leaving it charging on fastboot, but that did not work, I tried flahing a rom from recovery (wich said I had 47% battery) and it successfully did but when trying to reboot to system it again showed no battery logo. so I put it on fastboot and tried flashing MIUI with Miflsh, did not work but it told me the ROM was bad (same one I had use before) I made sure to extract it again and again nothing, I assume it was the antirollback, so I downloaded the next one on line 12.5.6.0 (I was on 12.5.4.0) and that finally worked (Miflash actually gave me lots of errors but you can google them and most of them have easy fixes, like creating a log folder, etc)

that was yesterday, after it worked it showed me the phone had around 60% batterywich lasted around 30 minutes to 40%, then it took 10 minutes to spend the next 20% (it told me that) and then it drained 1% every second (Im not exagerating I thought I had fried my battery with all that power button pressing) but I had hopes of it just being a configuration issue of the phone not knowing how much battery it had so I just left it power of and pluged it then so the phone knew what was his zero, now the phone as been on 100% for like 2 hours (sharing internet), so yeah, I think the battery has some issues but its better than nothing so Im happy with that.

that is my whole experience, I hope it helps you, see if you manage to get any response from the phone and if you do you can try contact me for help in any point along the way.

Update, it just got to 99% after 2 hours 10 minutes aprox IDK, I would need to test more to be sure