Further investigation showed me that it has to be a bug.

I tested with Dovecot 2.2.36.3 (a7d78f5a2), Pigeonhole version 0.4.24
(5a7e9e62):

In this version the additional addresses in vacation :addresses
["test@example.com"] are handled case-insensitive.

In the new version: Dovecot 2.3.7.2 (3c910f64b), Pigeonhole version
0.5.7.2 (7372921a) installed via https://repo.dovecot.org/, (same sieve,
same configuration) these addresses are handled case-sensitive.

The case-sensitive matching of mail addresses, doesn't make any sense to me.

Could someone confirm this behavior?