I guess there might be some Maven problem due to the fat jar including some dependencies that you already have installed through Maven
Yes, that's the problem I have. And also the fact that this fat-jar is updated all the time and I can't set a specific version (for example if the new one has some bug or introduces a check I can't fix right away or something). And that's why the Maven artifacts are nice. But isn't the process for releasing the fat-jar similar to the one for Maven Central? I also see there is an artifact in GitHub packages - https://github.com/validator/validator/packages/892686 - I could in theory use that one (I guess it's the same as in Maven Central) - so perhaps you could at least keep this one up-to-date?