Filters Are Acting Very Weird


So today I started development for a client of mine but then I came across something very weird. The issue is as follows.

I started with adding a few filters but than I wasn't able to select multiple filter variants (e.g. color) whilst in a different category I was able to do this. And I checked this multiple times (even in the database) and the properties are the exact same. Both of the filters / properties are the EXACT same except for a few variants.

So what is causing this effect? The filter types are both on multiple (defined in the features) but it doesnt allow me to select multiple?

After digging a bit more I also decided to make every filter either text or multiple. This also did not fix my problem. Does anyone have an idea on how to fix this filter behaviour?