If you create a new group, it’s not available for selection on the permissions page of the Category edit popup until you reload Discourse.
It looks like this was discussed way back in 2015:
https://meta.discourse.org/t/custom-group-not-available-for-permissions-on-new-category/29040/4
…with a link to a non-existent bug report – so this is a fresh, existing bug report. 
Never been able to reproduce this. Are you on master? Can you give exact steps please? A video would be perfect.
Hmm, I just went to record this, and was unable to reproduce it – but it definitely happened to me yesterday immediately before I posted this topic. 
Maybe a race condition somewhere would explain this strange behaviour?
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I tried many things in different browers, if you ever find a way to reproduce, happy to fix it.