Stop the dock moving between monitors on macOS - authohide-delay not working

I know I’m not the first person to ask how to stop the dock from jumping between monitors, though Apple doesn’t seem to realise that it’s a popular request. Most of the usual suggestions aren’t really practical for me, but there is one tweak which is supposed to work:

defaults write com.apple.dock autohide-delay -float 9999999; killall Dock

In fact, I did that on my Mac Mini some time ago, and it’s good. I’ve finally got a second monitor for my MacBook Air, and I tried the same trick. However, this time it isn’t working. I even restarted (though killall Dock should be enough), and it still doesn’t help. Both my Mac Mini and Macbook Air run the same macOS: Sequoia 15.5. They also have the same settings for Auto-Hide in the control panel. The only thing I can think of is that I applied the setting on the Mac Mini before the Sequoia update, but I can’t see why it should still be working if I can’t use it in a later version, so I don’t think that’s it. I know there are many questions related to this but (a) they’re old(ish) and (b) the ones that answer the question seem to settle on the autohide-delay setting. Has something changed, or is there another setting I should apply to at least delay switching the dock to another monitor?