I noticed that in Google Messages, I was able to verify the other persons keys via a QR code in Android System Key Verifier but for the other person it didn't work. It did always fail with something like "key couldn't be verified".
When trying to manually verify the key is the same, the numbers were different.
Does that mean there's a MITM attack or is Android System Key Verifier in Google Messages not reliable?
Maybe someone can try to reproduce this?
Edit: I installed this app by Google for qr code verification, but the key was also different when I tried verifying manually without this app:
https://play.google.com/store/apps/details?id=com.google.android.contactkeys&hl=de