Calling a foul is up to the discretion of a referee. If he's not calling it then it's not happening.
I think one of the reasons the holding Wlad was doing wasn't called is that it isn't a kind of foul that further endangers the life of a fighter. It's not like a rabbit punch, a low blow, headbutt, or following through with an elbow.
I don't fault Joshua for holding and hitting at all...when you want to do damage to someone you'll ATTEMPT anything, he corrected himself after the warning.
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