Mail app suddenly marking legit emails as Junk

After years of flawless behavior, Mail (v13 on OS X 10.15.3) has started marking more and more legitimate emails as Junk, and placing them in the Junk folder.

This is strikingly new behavior, it's been going on for the past 4-6 months, getting worse over time, and there's been no changes in emails, no new email addresses added, no changes in settings in Mail or over at Gmail.

I've always been extremely responsive to immediately adding new legitimate email addresses to my Contacts application, adding the address, the proper name, business, checking spelling, even adding a photo image to 90% of them (not that this has any bearing). I'm also a Zero Inbox follower, so I'm constantly keeping Mail light on its feet, never more than say 15 emails in the Inbox, none in Drafts, I monthly remove large attachments, delete emails older than 18 months, etc. Again, though, none of this is new behavior. I haven't altered anything in the past 6 months since this started happening.

And these emails that are being caught in the Junk folder are from people/email addresses that I communicate with regularly.

I used to always keep Mail's Junk filter off, since all my email accounts are Gmail, and I'd always heard that was the better way to go. But since this issue, I've tried toggling Mail's Junk filter on and off, to no effect.

Anyone else seeing this new behavior? Any suggestions on a fix?