I can’t reproduce, the order saves fine for me on 5.1.9, windows.
Find the pcbnew preferences file and check if it has right permissions or if it gets updated. On windows it’s in %APPDATA%\kicad, not sure about mac.
The line that stores button visibility and plugin order starts with ActionPluginButtons=
Why not?
The MAC platform is under-tested compared with Windows and popular Linux distributions
You help the developers with bug reports, you get a better version next release. It is a good way to pay back for a free tool.
I think this has to do with the app translocation “security” feature that macos added not so long ago. It probably changes the generated path of kicad.app folder on every launch and therefore settings stored in the preferences become invalid as they store virtual paths.
Try using another folder for plugins, one that doesn’t get translocated. You can find all plugin search paths by running this command in pcbnew scripting console: