> This was not a bug after all. But would it be useful to mention in the
> doc string that the variable's behavior will be different if modified
> by setq? Or is this already common knowledge? 

Thank you for the suggestion to mention this in the doc string.
While updating the doc string, I noticed the problem that currently
it doesn't update each frame individually according to the new
customized value, so this problem will be fixed with this patch: