Non-admins can not see tag usages for categories they have access to

This is a very weird phenomenon to me: Tags only used in non-public categories are not shown on the tag overview, even if you have access to the categorie, unless you are admin (or maybe moderator). This seems like a bug to me.

Check the tag permissions. It’s possible to create tags that are usable by only certain groups. It’s almost certainly not a bug. See Admin guide to tags in Discourse

Tags on private categories are hidden for normal users from the tag page by default.

Maybe you want to change the include secure categories in tag counts site setting