I believe this is a case of vanilla Minecraft behavior with the empty bucket acting on cauldrons like if they were vanilla cauldrons.

Please post a copy of your config, I need to know if you disabled any features which might affect this, as in my testing any fluid worked fine

Yep, nevermind, the issue is totally on my end. It was a conflict with RedstonePlusPlus which adds these interactions (filling of water bottles and buckets), disabling those in the configs did the trick.