And for the life of my I can't figure out why. I don't use it often, so I'm not sure if anything changed, though as I was searching for a solution I found T243101. Dunno if that's related.
For example: https://commons.wikimedia.org/w/index.php?search=deepcat%3A%22Mike+Pence+in+2020%22&title=Special%3ASearch&go=Go&ns6=1 returns 11 files. Some from https://commons.wikimedia.org/wiki/Category:Donald_Trump_rally_in_Wildwood,_New_Jersey_(January_28,_2020), some from https://commons.wikimedia.org/wiki/Category:Mike_Pence_in_2020 and a few others. It's unclear how these are selected, but https://commons.wikimedia.org/wiki/Category:Mike_Pence_in_January_2020 alone contains 188 files so obviously 11 is not enough.
https://commons.wikimedia.org/w/index.php?sort=relevance&search=deepcat%3A%22F.+van+der+Kraaij+Collection%22&title=Special:Search&profile=advanced&fulltext=1&advancedSearch-current=%7B%7D&ns6=1 returns 248 results, but https://commons.wikimedia.org/wiki/Category:Images_donated_by_F._van_der_Kraaij alone contains 462 files.
Documentation at https://www.mediawiki.org/wiki/Help:CirrusSearch#Deepcategory is virtually non-existent. It says "The depth of the tree is limited by 5 levels currently (configurable)" but how, where or who can configure this is a complete mystery. Even if the issue could be resolved by adding undocumented parameters, that would mean the defaults are bad.