SOLVED:
With help from user 0xpab they mentioned it could be just one or two addresses that cannot receive ERC1155s for whatever reason.
I decided to cut my airdrop address list in thirds and then went one by one deleting addresses in the Manifold airdrop list and hitting the Airdrop button to find the culprit and indeed it was a random collector’s address. No idea why it would do that with theirs but also I found another one that was the Foundation deployer address. So that one makes sense why it cannot receive the airdrop.
I agree with 0xpab and wilkins.eth it would be great if there was a warning or highlighted box or something to show which addresses don’t work in the Manifold airdrop mechanism. Cheers