foreSurName wrote:
Running Mac Mail Version 16.0 (3864.200.81.1.6) on Tahoe 26.1 on MacBook M1. English is UK, therefore 'Bin' for 'Trash'.
Mail directories 'All Bins' and 'All Junk' are not around in Mail below 'Favourites' initially.
After having added them to Favourites, they will have disappeared from there when quitting and restarting Mail.
Reason and repair?
Seems odd.
This happened going to the 26.1 point-update(?) or the Full upgrade to Tahoe...?
This is the first report I have heard either as a Tahoe macOS 26.0 upgrade, or point-update 26.1
You can try quitting Mail.app and relaunch it holding the Shift key down and compare your results.
Then add back in your Favorites:

no resolve— you can delete the mail account and add it back in to sort issue:
Remove or temporarily turn off email accounts in Mail on Mac
Add email accounts in Mail on Mac
Note—
IMAP accounts stay on the server, so no local loss, when you re-sync
POP accounts download and remove from the server—therefor save your inbox/sent box if you want them
“On My Mac" is just that, local folders you created and moved mail into. So no loss
To be proactive you can file a bug report / submit your feedback direct to the product Team here:
Feedback - Mail - Apple
Mail.app is baked in to the macOS— you can reinstall the macOS on top of the existing macOS as an option.
How to reinstall macOS Recovery (both M1/M2/M3/M4 and Intel) — How to reinstall macOS - Apple Support