Question about OT friendship legality

[JLSgolden]

Say I create a Pokemon in Fire Red with maxed friendship and drop it into an Ultra Sun save. Under OT friendship it then shows me the base friendship  for the Pokemon, however, I can change the OT friendship to max again and it wont be flagged as illegal/fishy. I figured that these contradict one another as one hand it resets OT frndshp but on the other hand it doesnt flag anything if I change it back to 255? What exaclte do the games "think" happens there?

 

Asking because if I transfer a max level pokemon it would be very much fishy if the OT friendship is at a base level but the Level is 100.

What I also noticed is that if I create a virtual console Celebi it then will flag OT 255 as illegal because it "doesnt match event base friendship" which then completely confused me as how would it know that I didnt max out its friendship on OT? Also, why does it matter on virtual console specifically but not on gen 3/4 games?