The two most likely possibilities for this problem are: bad mag or bad ammo. I swapped my carry mag for the spare (never the training mag) and now the one that jammed up can be used for training and hopefully will help me identify the problem- obviously if the mag causes a FTF with my practice ammo I will know what's wrong.
Today I fired 21 practice rounds through the mag hat participated in the FTFs last weekend with zero failures (the streak continues!). For now the mag seems to be ok… so what about the ammo?
I discovered an interesting difference between the PDX1 ammo and my practice loads: the PDX1 ammo is actually much shorter (1.119″ avg OAL) than my practice ammo (always between 1.125-1.135″ OAL). Some firearms have tighter toerance than others- I suspect this Walther falls into the 'tighter tolerance' category.
Resolution: try longer ammo. I found some Hornaday Critical Duty ammo that had about half the rounds masuring at 1.130 – which is my target OAL for practice loads. Luckily there are a few spares in the box so I can test my theory… if there are failures I will report them.
After experimentation at the range the Hornaday Critcial Duty ammo fed flawlessly through the PPS- regardless of OAL. The PDX1 ammo has a more jagged edge to the projectiles while the Critical Duty is more rounded and consistent- that could also be hanging up the rounds in the magazine.
I'm going to avoid the PDX1 rounds from now on. And keep testing every 8-12 months.