No idea but since it’s something which appears unable to work much above ground level I wouldn’t worry about it. You could try going out at night and seeing if you can spot the culprit.

Your plant need liquid iron there no insect problem it iron low holes come in older leaves. look leaves you yellowing that iron problem apply iron next growth be yellow free. iron work now forward in berries the drupelet show iron need as white where should be black., any new planting of blackberries will need iron applied because less roots

If it’s to do with iron why is it just the lower leaves? And why are some leaves completely skeletonised? I’m highly sceptical about that theory. Btw the wiggly track is leaf miner, but that didn’t make the holes.

The yellow showing leaves that fall out as hole sometime it can be tip are half leaves, damaged leaves are oldest ones on plant iron want make any repair to leaves just make new growth perfect. If this plant don't get iron it going produce white drupelets on berries in future.