Note that currently with ULA if you have dual-stack IPv4 will be given priority over ULA. There is a late-stage—Submitted to IESG for Publication—draft that will change this:

* https://datatracker.ietf.org/doc/html/draft-ietf-6man-rfc672...

More than just IPv4 priorities, almost all other IPv6 addresses are given higher priority which makes routing between ULAs on an internal network problematic.

That draft doc seems to fix multiple problems at once.

Ah, thanks for posting that, I was wondering how things were going there.