Steps to reproduce:

I have a GPG public key that should expire in the year 2030. The output of "gpg --show-keys" confirms this. All Thunderbird versions up to the 91.7.0 (including) show the same correct date. Version 91.8.0 started showing the key as expired.

Actual results:

Last working Thunderbird version: 91.7.0
First not working Thunderbird version: 91.8.0 (also applies to 91.8.1)

These are the versions I tried:

78.14.0 ok
91.5.1 ok
91.6.2 ok
91.7.0 ok
91.8.0 fail
91.8.1 fail

As usual, only after submitting this I discovered bug 1763641.

The reason is likely a SHA-1 signature in your key, we intend to restore the use of SHA-1 in key binding signatures in bug 1763641.

Please let us know should your bug not be fixed by 91.9.0, which will be released soon (probably later today).