I selected "Plaintext" as a default paste option in the Clipboard manager. This works when I select the "Standard" button in the main Clipboard manager window.
I also selected "Paste selection when triggering the clipboard manager action again while it's already open". When I use this workflow, it does not use Plaintext.
MacBook Pro M1 Max with MacOS Monterey 12.3 and BetterTouchTool 3.756 (1897).