I think I have found a satisfactory solution after years dealing with this issue. Not sure if this is a new option or one I never noticed before but I turned off toolbars on other monitors so that only the main one has the toolbar and now it autohides as expected and has been working without a glitch for several weeks no matter if I have a second monitor or not. It's only slightly inconvenient having one toolbar but definitely worth it not having to deal with the toolbars suddenly not working or autohiding, or just switching which monitor it works on.
Let me know if anyone else has tried this and it works ok.