Shared Mailbox some emails going to Junk Folder

We had this shared mailbox that some emails going directly to the junk folder. Each member who has access to those shared mailbox does not have any rules that will or might cause it. Likewise, the shared mailbox itself does not have any rules. This happened all of a sudden and for unknown reason just about not more that 5 months ago. Still, we can't figure out why. I am hoping maybe someone can give us some clues or advise what other things that we need to check.

Thank you,
Warren