[lineslinger]

It takes a lot of force to snap off the stud as shown in your photo.
I guess it possible the stud itself is at fault but I have doubts about that.
Something is obviously binding or an out of sequence opposing force had to come into play for that stud to snap.
The curious part is the push rod does not appear bent although the photo does not show the complete length of the rod. Looking at the ware on the push rod from rubbing against the guide hole (which it should not be doing) in the head.
The push rod might have gotten hung up/stuck on the newly worn edge (at least it looks like a sharp edge) at the top of the guide hole and forced the stud to snap with an out of time/sequence lift. One could speculate the rocker arm might go before a valve train stud though. Just one quick guess.
Some mods to assure clear push rod movement on all cylinders are needed, obviously, along with confirming the push rod length being correct.
Too bad but in a way you may have gotten lucky that the stud let go instead of a more involved and expensive "injury".