There are examples of dialogues working in different ways in the past - so what really needs to happen is a decision should be made on how action buttons for a modal should work and this should apply to every modal in Moodle with some useful checking of many different modals to confirm that this is the intended behaviour. 

MDL-62160 is related in that this issue changed the behaviour for everything to comply with a quiz specific modal - but this affects all of Moodle and needs proper discussion and agreement. 

Here is an example of a badly displaying modal which was fine before.