Even though Creative Core isn't required, some modded clients are prevented from joining saying they don't have Creative Core installed. Which is weird because I can join on the same server with just a vanilla client with zero issues. Seems quite random.

In the logs it complains about something related to the resource packs the client is using(?) I would give you a log file but for some reason all of my log files were cut so yeah.

you say about logs... can please upload those?
serverlogs and clientlogs please

Sorry I'll try and get logs for the error when I'm able to get someone to help me test things again

If CreativeCore is installed on server side, the client requires it as well. The other way around this is not necessary.