Hi @Daniel S ,
Thanks for you reply.
According to my search, this is currently no official document saying the known issue about Shared Mailbox search and FSLogix.
As far as I know, when you are in cached mode and using the Outlook built-in search, by default, Outlook only searches your Current Mailbox.
So it is suggested you change the search scope to Current Folder when you search for some folders of shared mailbox.
Hope this can help you!