Seems reasonable: Gmail's Javascript makes the message appear immediately because it knows it will appear there. You delete it, but that command has no effect b

Seems reasonable: Gmail's Javascript makes the message appear immediately because it knows it will appear there. You delete it, but that command has no effect because the real email message is still being relayed to Gmail's mail servers and getting redirected to yourself, but nonetheless the message vanishes from the UI. Then it arrives. I'll not try to reproduce. This is not a bug.