OK, in that case, I have a similar situation going on with onion sets (tiny onions planted like bulbs but shallowly so the topknot is above ground)
In my case, the culprits are robins and blackbirds (thrush family birds). I believe they see the topknot of the unsprouted or barely sprouted onions and think they are worms or slugs or something. They yank the onions out of the ground and leave them lying next to their holes, new roots drying in the sun.
They scratch around the mulch and soil near the holes, tossing loose mulch and weeds around. I end up with uprooted onion sets and new seedlings buried under mulch flung around by these birds. I've been forced to patrol twice a day -- in the morning and late afternoon -- to put everything back in place.
I can't wait until the onions grow enough roots and tops and the seedlings to grow enough not to be buried.
Does this sound like your situation? I know the blackbirds are great slug and snail hunters so I generally LIKE that they're in the garden.... Ah well. I was advised to put birdnetting over the onions in case you want to try that.