This seems to be solved for me as of August 11, 2023 update to 11.6.
I'm having this issue, too. Same sys info as Todor except my DE is Gnome. Also (maybe separate?) Pamac always shows the cache size as zero even though there are plenty of files in the cache. pacman -Scc works.
Edited to remove my ignorance: Once I edited pamac.conf to lower the number of versions to keep the CLEAN button came to life.