Smart mailbox for unread messages in Mail.app

Problem = I've created a smart mailbox in macOS Mail for "Unread" messages with the following rules: > Contains messages that match all of the following conditions: > > - Message is in the mailbox: Inbox > - Message is unread The smart mailbox will always *correctly* include *unread* messages in my inbox. But often, it will *incorrectly* include *read* messages in my inbox. Any ideas on how I can fix this? Other information and troubleshooting steps so far = - Currently using macOS Sierra 10.12.6 - Email account is hosted by Google - Other smart mailboxes with other rules still work properly. - If I use the filter button in my inbox, Mail will properly perform filtering -- even for unread messages. - I tried marking the problematic messages as unread and then as read again. Afterwards, the number next to the title of the "Unread" mailbox will decrease (because I have taken steps to mark messages as read), but the "Unread" mailbox itself will still list those messages (even though they are read). - I tried the above step from gmail.com. Same result. - I tried getting new mail. Nothing changes. I tried rebuilding my mailboxes. No rebuilding happens. - I tried rebooting my computer in safe mode. Then, any smart mailbox I created would contain no messages regardless of the rules I set. - There doesn't seem to be any pattern to which messages are problematic. - My "Unread" smart mailbox still behaves properly for non-problematic messages even if it contains problematic messages. - Once a problematic message appears in my "Unread" smart mailbox, it will stay there for a few days. Then, it will suddenly leave "Unread" and sit in my Inbox where it belongs.