I was thinking of asking exactly the same question as I have had the same experience. But yesterday, on pulling a few more twisted leaf onions up and looking more closely, I found just one leaf miner bug of the brown variety.
After doing some more reading online, I wonder whether it’s possible that after this stage of pupating they might drop down into the soil and wait there until they fly again in September to October. Could it possibly be that many have already dropped? I’m still rather puzzled though because I can’t see evidence of tunnelling on mine. But this would be my first meeting with these creatures so I haven’t got any previous experience to draw on.
I’m certainly going to cover my leeks though, to defend against the possible later September/October infestation, and keep my fingers crossed that they weren’t already visited when I planted them out in the second week of May.