I can confirm this

Tests here show that this happens ONLY when the pointer is focussed inside a field, whether the menu item (and anything else in joomla back-end btw, each redirecting to different pages) is new or not.
When it is not new, it checks out the item.

Afaik enter/return has never be equivalent to Save in J. It only works for Search.

It "could" work with a supplementary js targetting keyCode == 13 for keydown for the Save button I guess.

Correct behavior for Joomla admin forms, when you push Enter:
nothing happens

Because we use JavaScript Joomla.submitform to submit the forms in administrator side, when user explicitly pushes the Save or Close or any other button.

Described here issue happens due <button> tag in new modal markup, that inside our <form> , that make <form> behavior like described in the first comment here.

in theory, the solution is to add type="button" (like <button type="button">...</button>) to all these buttons in the modal markup, or replace them to simple <a> tag

Fix is more hard than I expected ... need to search all call of JHtml::_('bootstrap.renderModal',... and edit 'footer' => '<button ...
Hope there will be someone more brave than me

@Fedik If will not find anyone brave the next few hours, I can handle this. Currently in Poland it is 4 in the morning and I have to rest

@Fedik @brianteeman @infograf768 @uglyeoin please test PR #8874

@wilsonge Please close as #8874 is marged

Ty!

Thanks