[Huawei P30 Pro Questions & Answers] Dominiko:
EDIT 2024-04-08: I am devastated to report that after almost 2 years of working flawlessly since the repair, my phone again stopped seeing the card, without me ever removing it. I restarted the phone and I see the card again for now, but I have no doubt that the issue will keep on coming back, ever more frequently, as it did 3 years ago. Time to look for a new phone :(

EDIT 2024-04-21: As expected, the card kept breaking down, so I decided to clean its contacts with some WD-40, as seen on a YouTube video about repairing Micro sd cards that phones do not recognize. So far it's been 20 hours and everything is working, but this method is quite extreme and you might damage your card and/or phone in doing so, so be advised. Alternatively you could try sticking a piece of thin tape on the back of the NM card before inserting it in the phone, so the contact between the card and the phone is tighter. Could work.

EDIT 2024-04-24: Both WD-40 and tape tricks stopped working soon after restarting the phone. I officially give up. I would need to send the phone for official repairs again (see non-bold text), but it's probably not worth it anymore, since my phone is 4 years old at this point and the model is 5 years old. It's a shame that my relationship with this otherwise excellent phone will have to end on a bad note because of that dumb NM card. Wish I went for 8/256 variant all these years ago. Targeting to buy Pixel 9 Pro this autumn, we will see.

TL;DR: If you're having issues as in the topic, it's very likely that this is because of your P30 Pro's hardware fault, not your card's

I finally have an update on this topic, it's probably going to be a rather long post but the story was quite a rollercoaster for me.

I was living with this nm card issue ever since summer 2021 and all the time I was hoping that this problem was either because of some kind of a software glitch which EMUI 12 was going to fix, or because a fault with my nm card. However, on late April 2022, even though I knew my phone was weeks (or days, even) from receiving an EMUI update, I stumbled upon a good offer for a new, genuine Huawei 128 GB NM card, and I decided not to wait for the software patch and buy the Huawei card for a good price (I was using a "shady" Lexmark 256 GB from Aliexpresss), to at least verify that the issue was not because of my card.

Unfortunately, when I received the card it turned out that it behaved exactly the same as my Lexmark: working perfectly fine when inserted into a USB NM Card reader, but not working at all when inserted into the phone.

After that fiasco I decided that enough was enough and I took the phone to the nearest, official Huawei customer service centre. Turned out that the warranty had just expired before I arrived there, which made me furious for waiting for that software update for too long. Either way, I handed the phone over for repairs, and the dude who took the phone told me that the issue I was having was either because of 1) a fault of the tiny motherboard on which the nm/sim slot is installed, 2) a fault of the data cable (tape) connecting the tiny board and the main board (both cheap and easy repairs), or 3) the "main" motherboard, which would make the repairs unfeasible due to costs.

A couple days later, I'd received a call from the customer service, and the lady broke the bad news to me: they had replaced the tiny board and data cable (tape), but the problem was still there, so it had to be the main motherboard's fault, repairing which would cost me almost exactly the same price that I paid for the entire phone 2 years prior! I refused to have the mobo replaced, but at least the service said they wouldn't charge me for any money since they couldn't fix the problem, and the new parts would stay in my phone since they would rather save time by not putting them back out than to save their own parts for later.

When I arrived at the service to sign some papers and get my phone back, the lady behind the counter said that the slot (card) seemed to be actually working at the moment, but that was only temporary and it was going to disappear again soon.

Now get this: I kept using my phone normally for the next few days, and the issue of disappearing nm card would not appear again! After 3 days I got my EMUI 12 update ready, so I installed it and had my phone factory reset, as it's recommended to do when you install a big update on your phone.

I've been using the phone with EMUI 12 for over a month now and my Lexmark 256 GB from Aliexpress is still working just fine, and (not to jinx it) I'm assuming that everything is permanently fixed from this point.

So, in summary: to have this "disappearing nm card" issue fixed I took it to an official repair centre, had replaced both my mini motherboard with nm/sim slot attached to it and the data cable (tape) linking the tiny board with the motherboard, and then the issue disappeared - but strangely, only about 24 hours after replacing the parts.

So, for my last words of advice: if you have issues with your card as we do/did in the topic, don't count on some magic software trick that will help you: have your phone repaired instead.

PS If anything (god forbid) breaks down with my NM card slot again, I will let you know