Strange, I retested and it worked.
This is my filter, testuser1@domain2 is a local user mailbox, markus@domain1 is a mail user on another server.
In the logs I have following entry that shows that the mail is forwarded:
2025-05-13T08:48:32+02:00 [1:mail1:dovecot] lmtp(markus)<184826><CXs8AT3rImj60QIA0NR6rg>: sieve: msgid=<d69ad9894da6367d47e5cdf8e3b9e0f1@domain1>: redirect action: forwarded to <testuser1@domain2>