Split button is basically a button with a default action and several other actions which are shown in a drop-down menu.
While there's no doubt in how it functions, the issue is with how the default-action and more-actions icon should be highlighted in UI.
I have created basic variations of split buttons in below pen.
-- edit -- Adding GIF ..
In the above, both styles 1 & 2 are properly highlighted as a single group. But with style 3 user might mistake the more-actions arrow as separate button.
So the question is whether style 3 is valid or not ?