Okay, settings both robot settings to “translate” worked. Now for both translated pages the settings are saved in the meta table, instead of wiping one page’s setting when updating another.
Still don’t understand what “copy” should mean in this particular situation though.
So if I understand correctly, when the property is set to “copy” (which noindex and nofollow are) the settings should be synchronized between translations. This would mean that changing the robots in one of the translations should change it at the other(s). Then it’s pretty weird this doesn’t happen in situation 2.
But thanks for pointing me in the right direction.