Because menu settings overrule article settings here. You could use option Use Article Settings for Show Intro Text if you want to set Hide explicitly in article or set Hide in menu item directly.

That's exactly what I do: manually 'Hide intro' in article settings. (Default in article settings is Show intro.) But, as soon as I link the article to a menu, the 'hide intro' setting does not work anymore,
I do not know how to change menu settings. Still, I think the article setting ought to override this.

I do not know how to change menu settings

Please ask in a forum. These are Joomla basics.

Forgive my being persistent. I am not looking for a way how to change any settings; I am reporting a bug. And a bug this is; it has been discussed in more Joomla Forums, i.e. forum.joomla.org. Please check out yourself.

It's the expected bahavior:

menu settings overrule article settings here

Otherwise options for articles in menu items of type Single Article or Category Blog would be complete nonsense/senseless/needless.

Provide detailed testing instructions if you think it's a bug. What menu type, which settings and so on.

You are right, bertmert: no bug. thanks for helping me out.