The EUPL obligations still apply, unless they specifically conflict with the other license. There is some dispute on how exactly that is to be interpreted, but hopefully it will be clarified in version 1.3: https://interoperable-europe.ec.europa.eu/collection/eupl/di...
A “conflict” here is intended to mean that following some EUPL obligation (such as publishing the derived source code) wouldn’t be allowed under the alternative license.
I don't understand how that's supposed to work - then you can't distribute under the compatible license; only under a weird hybrid of that license and non-conflicting EUPL terms. Seems to defeat the purpose of the compatibility clause?