Progress on this (no success yet): tested in 5.3.4 and unity 2019, bug does not occur... I jump to Unity 6, and not only is pausing busted, but the OnComplete event won't fire off at all, even when not using pauses. Something between there must be the cause...??
...And while testing why OnComplete wasn't being called... it's suddenly working again and pausing is back to working as expected...? This is a weirder issue than I thought...
EDIT: might not be related to OnComplete at all, I may have messed up a debug thing... it's weird that it *is* fixed for me now though, since I could swear I was getting the broken pause behaviour