M1 - Boot partition suddenly doesn't show up anymore

[Polio23]

Hello,

I have 2 partitions with two MacOS versions. That worked pretty fine, but suddenly one of those partitions simply doesn't show up anymore when I boot my iMac(m1). Only one of those partitions show up and I have to choose it. In disk utility the missing partition still shows up, its still there.

Anyone has an idea how to fix this?

 

[BanditoB]

Likely, that partition got corrupted somehow, so is no longer bootable. Have you tried running First Aid on it? If that doesn't to anything, you may need to reinstall MacOS.