This happened to our motors as well but it should be okay to use still. Just make sure it is aligned properly before you really tighten it down and I would recommend using washers on the screws just to make sure they won’t loosen.
To align the motors well our team has been putting the axle into the motor and screwing in the one hole that actually works so that the threads will still line up with the holes in the c-channel or whatever you are screwing into.
This issue with broken motor inserts was discussed back in this thread:
If current inserts are unusable you could replace them with the standard 0.5" standoffs, but that could put motor plastic base under additional stress, which could break it in the worst case.
I found a replacement part that is functionally similar to the vex motor inserts but has stronger walls, since it uses 6-32 screws:
Some people may question its competition legality, to which I say that until VEX sells motor insert replacements at a reasonable price or exchanges broken ones free of charge, you should not feel bad about using a third party replacement part.
Unfortunately, I can’t, because I am neither primary nor secondary contact for our club and, also, I don’t think GDC will feel comfortable answering this question.
Same as with potential ruling on partially threaded screws and 393 output gears I wouldn’t want to ask the question if there is a chance they will discard a precedent of those parts being allowed in previous seasons, and rule them illegal going forward.