we noticed the exact behaviour on Kamailio 5.5.4 and topos.

If we disable topos the UPDATE messages are being recognized as loose in the WITHINDLG route and everything works.

When topos is enabled the UPDATE messages fail the loose_route() check and even if we force the record_route() the message is still not being forwarded.
I had just posted my own thread here because I didn't see this one.
I'll proceed opening an issue on GitHub so we can track this.

Best regards

Simone