1 minimal tag from group for a topic not enforced here either despite countless tinkering, recreating group etc… 
OK my bad, I was testing from an admin account 
Note that the “minimum tag validation” on a category will not happen if: tagging is disabled globally acting user is staff acting user does not have sufficient trust level to use/add tags
from How can I make the "tags" as REQUIRED when create a new Topic - #2 by techAPJ