iMessage Group issue with removing Contacts

My daughter has a project group chat set with a list of school contacts in it. One of the other members removed 3 contacts which is fine as they no longer need to be in the group. However they reappeared back into the group saying that my daughter re added them in. She said she did not re add them back in. The other group members told her off and she is very upset. I want to clarify if this is a known issue? iMessage randomly re adding previous added members. Another problem she noticed that sometimes she replies in the group chat and it appears a lot later then when she sent the message. She was using an iphone 6 with the current operating system and an iPad Air 2 with current operating system.