HTML Rendering Broken in Mail

[Voenix Rising]

Since I upgraded to Sierra, it now appears that HTML randomly does not render in Mail. Everything received since the upgrade is downloading plain text, even though when I log into icloud.com the HTML is obviously there.

I've deleted my iCloud account in Mail and recreated it. Same problem. I've deleted com.apple.mail.plist from Preferences (it hasn't recreated itself, which in itself I find kind of odd).

Any other suggestions?

 

[Voenix Rising]

Update: This seems to have been resolved with the 10.12.2 Beta (16C48b), but I have no faith that the next update won't break it again.