Thanks for discovering that and living up to your "Expert" title!
I have no doubt that it's a viable solution, but it's not quite what I was hoping for. As far as I can tell, any changes made to native Archicad 25 Library Parts cannot be saved back to the library part in the Archicad Library, as far as I can tell. I could save it as a new object elsewhere, but I'm not keen on creating confusion for the sake of a single checkbox.
I'll lay this on Graphisoft as a bug, and hope it's fixed in a future update (like all of my concerns).
Thanks again, Steve, for taking the time to root out the cause of this issue, I really appreciate it.
AC 6.5-26; macOS Ventura; 2019 MacBook Pro 2.4gHz, i9