Seeing that Outlook (2019) is syncing new messages, but only for the Inbox folder. All other folders do not seem to sync new messages. The messages that are in the folder at the time Outlook does the initial account creation/download of emails is what you get. There is just a message on Outlook saying "This folder was last updated at xx:xx" instead of saying "This folder is up to date" like it does on the Inbox.

Some custom folders will say they were last updated 2 minutes ago, some will say they are up to date, others will say they were last update 20+ minutes ago. Others will say they were last updated x minutes ago, but do not contain emails that were sent before the last update time.

Restarting Outlook does nothing to fix it either.