External Storage lost from time to time

Since one or two OTAs I have a sporadic problem, that some apps loose the permission to write to external storage. That affects automatic backups from Signal and from my timetracking app.
Anyone else having this problems?

I have not seen that personally so far. You are on the stock FPOS, I assume?

I guess there are also no notifications about revoked permissions? The period for that is something like 3 months usually, I believe - and it should not happen if you keep using the app.
My best guess: The external storage might sometimes disappear from the system for whatever reason and that resets the permissions. But even that never happened to me as far as I can remember. Any other issues with the external storage that could be related?

Yes, I use stock FPOS (A12, latest OTA), but rooted. The notification from Signal claims that the external storage disappeared, even if it resides in the /sdcard mount, not the external SD-Card.

And it only happens to signal and my timerecording app, not to other apps that tend to write backups. Signal support could not help me either as they claim, this would only happen on FPOS.

And no, there are no other errors or failures related to storage on my phone