On my Mac Studio, “Move to Predicted Mailbox” was always dimmed for Gmail messages, even though Mail on my iPhone suggested the folders I’d expect. Re-adding the account, rebuilding mailboxes and resetting Mail’s index didn’t help.
I turned off Show in IMAP for Gmail’s All Mail label and restarted Mail. After that, predictions worked in two Gmail accounts. Has anyone else been able to reproduce this?
Hiding All Mail does have a tradeoff. Archived messages with no other label can stop showing up in Mac Mail. I used a separate _Archive label to keep those visible. I sent the diagnostic details to Apple as FB20180346. Recommend you mention this if you file your own feedback with Feedback Assistant, or use Feedback - Mail - Apple for general feedback. My setup notes are here if they help: https://mail-prediction-workaround.dave-mac.chatgpt.site