Outlook recipient will not receive meeting invites if multiple people are invited

Question

When I send out an outlook appointment to a recipient individually, they will receive. However, if I invite that recipient along with other users, she will not receive the invite. Once a multi recipient appointment is sent, I am, however, able to forward that meeting individually to that recipient. I have been sending invites to her for years without issues up until now. She has verified there are no rules or delegations or anything of that sort set up on her side.