Seems there is a problem when using filters on servers. After you set an item in the filter, mousing over it will delete the item. The item doesn't stick when you close the filter either, making it impossible to actually add anything to the filter.

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I can't reproduce the issue on my local server. What mods do you have installed?

I tried this on another server with no other mods and couldn't reproduce either. My main pack has a crapload of other mods, I'll try to determine what's causing it.

I ran into this issue today as well. It seems to happen more if I try to max a filter out. If I Keep it 6-8 items filtered I can get the filters to do their job.