The condition on the menu item options is "Install when..." which does indicate this is expected behavior.
As far i know, the menu aren't refreshed when changing data in a record, nor when browsing from a record to another, but only when changing the layout or, as you experienced, installing a custom menu set.
You can report possible bugs in Report an issue section.
I suppose. The difference between "install when" and "enable when" is subtle but significant; I was (obviously) looking for the latter --- something akin to conditional formatting or hiding an object. (That something evidently doesn't exist. I'll either live with the trigger --- unlikely --- or find another way to accomplish the same thing, like hiding a button on the layout.)