A broken trigger pin isn't something I'd call common, but neither would it be something I'd consider terribly surprising, either, especially in the models chambered in the harder recoiling calibers. It's not unheard of to hear of trigger pins breaking in the harder recoiling Glock .40/.357's. The pair of grooves are the weakest points of that pin, after all, and the .40/.357 guns experience more recoil forces than 9/45 guns.
If the G32 in question has ever been detail-stripped I'd have to wonder if the trigger pin had been improperly reinstalled, though. The trigger pin (meaning not just the locking block pin) has some stress placed upon it during normal operation (recoil), you know. If the slide stop lever isn't positioned inside the groove on the left end of the trigger pin, as intended, but rests on the larger diameter of the pin's shaft, it can create unintended stress on the pin during recoil (not to mention the potential for slide stop lever functioning issues to sometimes arise).
In my last Glock armorer recert class I remember the instructor making it a point to mention a few times that if we were seeing broken locking blocks, locking block pins and trigger pins that we weren't replacing the recoil springs often enough and that the guns (and those parts) were being subjected to unnecessary battering during recoil. (The recommended replacement interval in the class was mentioned as 2,500-3,000 rounds, which is more than the 1,500 rounds mentioned by the poster, I know. I just mention it to illustrate how recoil forces can act upon those parts.)
I remember examining a used G23 bought by one of our folks a while back. He said he'd fired it maybe 2,000 rounds and that the previous owner claimed to have fired no more than 500 rounds through it, as I recall. The only noticeably worn part I found when I examined the gun was the locking block pin. The finish was missing from half of it and it looked like it had beaten on by a hammer. The trigger pin still appeared fine at that point, and the trigger spring appeared to be the current revision, so I just replaced the locking block pin and recoil spring assembly and told him to go back to shooting it.
At 1,500 rounds it would certainly be a good idea to have the G32 mentioned examined by a Glock armorer so see if anything other than the trigger pin needs to be replaced to keep the gun in optimal operating condition.
Just my thoughts ...