Sounds like some app is messing things up once it’s got going.
Could you try using Safe Mode? This sort of temporarily simulates a factory reset but without the pain.
If Safe Mode allows stable microphone functioning then I would advocate making an exhaustive list of apps that are allowed access to the mics, and remove their permissions - one by one so that you can find the culprit. I suppose you could also work the other way: withdraw permission for all apps and then gradually grant again.
To list all apps with mic permission,
go to Settings > Apps > See all apps
choose an app that you know has mic permission
tap “Permissions”
tap “Microphone” > See all apps with this permission.
I have the same problem every few weeks (not hours) but I don’t use the mic much and only notice it wasn’t recording when I e.g. play a video some days later and find out has no sound. BirdNET sometimes doesn’t hear anything but I notice that immediately from the empty audio track. Only solution I know is to restart the phone.