How to fix joist hangers where joist is not fully on seat of hanger
Sorry if this sounds insulting, but that is terrible work.

Build a temporary wall underneath to hold the joists in place while this is being fixed.

Then remove the hangers, cut all the joist ends to 1-5/8" off the beam, stuff a new 2x behind, then re-apply the hangers. (Use plenty of long nails to attach the new 2x to the old beam.)