In the first pic it looks like the belt is rubbing the rear deck bracket, that can't be right. In the second pic after mowing the idler pulley is leaning, how floppy is the pulley and bracket with the belt off? Maybe the belt is climbing the edge of the pulley which pulls the arm tighter to compensate for the larger diameter and the bouncing of the arm as the belt catches the edge of the pulley is taking out the spring.
I replaced the pulley bolt on my tensioner this year, different deck, but similar tensioner. I had no idea it was like that, only removed it because the tensioner pulley bearings were shot. Someone had already sleeved the shoulder bolt hole that mounts it to the deck. I think you may want to inspect the tensioner.
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Bill