In larger verified chats (51+ members) it was several times observed that the newly added member can get message from non-verified senders before they see that they got added. This is probably due to the chunking of smtp sent out messages with classic providers (current limit is 50, after which several messages are generated). The newly added member can (or maybe even always?) does not make it into the first chunk, so that existing group members see the addition before the added members sees this addition.
This is probably best fixed by ensuring that added members are always part of the first cunk.