[Essential Phone ROMs, Kernels, Recoveries, & Other] sga999:
I figured out a couple of things.

1. I had said that "fastboot format userdata" fixed the Total Commander (or other File Manager) problem where it displayed 3 options and I couldn't click any of them. But that was NOT what fixed it. The culprit was that I had the LMT app installed (it's not in Play Store but on XDA. It's one of my

favorite apps that I've been using for PieControl for many years. It requires "Display over other apps". That is what caused the 3 options to not be clickable!

I can turn off that "Display over other apps" option for LMT (or just deactivate the app temporarily), and File Manager apps will have those 3 options clickable. That's not convenient but it works.

When I said stock Q (10) or Lineage official nightly didn't have that problem, I probably had not installed LMT. I can't be sure about that...I was doing too many things. So it's quite likely that I didn't care about installing LMT on those other installs.

About "fastboot format userdata", I had used that years ago when I couldn't get large files to flash. And since I did it again before seeing this Total Commander problem work, I made a poor assumption that that's what fixed it. Sorry!

2. Completely new discovery! This will probably not be interesting to you, @Tomoms, or most readers of this thread because it's about TWRP. But I wanted to post this anyway, just for documentation. Or maybe it will help someone (doubtful).

I like to install TWRP so that I can do backups. To do that, I install the TWRP installer zip so that it "combines" with the boot image. After that install, "in some cases", booting to TWRP did not work... it booted into bootloader instead. With lots of investigation, I discovered that the bootloader file abl.img was causing this!

The abl.img that is within the stock Q for Jan 2020 is "good". The one for Feb 2020is "bad". There are a couple of posts on XDA about the Feb one having a problem for both TWRP and Magisk. But since I was installing your ROM or Lineage nightlies, I didn't think much about that. However! The Lineage nightlies include that "bad" image! I don't know enough about these 2 bootloaders to know why the "bad" Feb one would be included in those nightlies. But since Lineage doesn't advise us to use TWRP, developers possibly decided to include the "latest" bootloader? There must have been no concern about this problem with installing TWRP with that bootloader.

The Lineage nightlies have abl.img (and many other images) in payload.bin. But yours does not. So if I had installed a nightly version before yours, the "bad" bootloader was installed. The same is true if I installed stock Q from Feb 2020.

To test the theory that the "bad" bootloader was the reason that booting boot/TWRP booted into bootloader, I tried fastboot flashing only abl.img. If I already had the "bad" one from Feb 2020 installed, booting recovery failed by booting to bootloader. Flashing the "good" Jan one fixed the problem. That is, I could reboot to recovery just fine. But if I reflashed the "bad" one again, booting to recovery fails and boots into bootloader. I can fix it again, merely by flashing the "good" bootloader. That is, there is nothing wrong with the combined boot/TWRP image, so it does not need to be reflashed. (For quite a while, I incorrectly assumed that somehow the TWRP install did not combine properly with the boot image).

Maybe there is some good discussion about these bootloaders and how flashing the Jan one would help with TWRP. But I sure can't find it!

Here are the names of the 2 abl.img bootloaders. I googled lots, but I could not find specifics about these or any possible newer ones. Maybe the latest is the "bad" one since that was the last official stock release.

Jan 2020 mata-b9326aa "good" one (stock release ending in 007)

Feb 2020 mata-34cc3f5 "bad" one (stock release ending in 032)