I have seen similar issues on occasion. I’ve had onions hollowed out from the inside leaving just a shell, like the one with it’s root-end facing us in the photo. I believe it was pill bugs and worms that ate out their innards; at least those are the critters I saw when I pulled one to investigate. There may have been rot involved first but I did not see any signs of it early on. There are some rots of onions/garlic which can cause similar issues too, and no doubt the various critters are happy to help out. Might be prudent not to plant onions/garlic in this bed for a few years.
The closest bulb, with a side chewed off looks like my onions/garlic when the gophers take an interest in them. Pocket gophers seem to be one of the few animals (other than humans) that like onions and garlics I don’t usually see rot at the eaten edge, but I typically catch the damage fairly early on and replace the bulb with some bait, but I suppose it would rot just from the exposure if left long enough. Regardless it does look like rodent damage.