A user reported some pages they added to categories didn't show up in the categories. I tried fixing it in all ways on their wiki but unsuccessfully. I tried testing this on publictestwiki.com and get the same problem (https://publictestwiki.com/wiki/Category:Testingissue https://publictestwiki.com/wiki/Testing_category_issue)
Could this be a caching issue? I will investigate.