How much experience do you have with shooting lightweight plastic-framed pistols, especially chambered in .40 S&W? "Limp-wristing" (or a grip technique/grip stability issue, as I prefer to consider it) can occur to different folks regardless of whether the pistol has a metal or plastic frame. The accelerated slide velocity and slightly "snappy" (muzzle whip) recoil of the .40 might exacerbate this for someone.
Have you cleaned out the shipping/packing oil from inside the magazines?
Is the pistol lightly, but properly lubricated?
Walther (like other manufacturers
) has continued to make refinements and improvements to their 99 series over the years.
They were able to identify and resolve a magazine body & follower issue causing some occasional feeding issues in the 99 series .40's back on the early 2000's. That was also about the same time their extractors benefited from a minor improvement (hook angle & polish).
Not being able to watch someone shoot the particular PPQ involved, or examine it myself, I can't know what's happening with them and their PPQ.
It's not uncommon to hear the expression (in some variation) that day in & day out, close to 95% of all semiauto pistol feeding stoppages and overall functioning "problems" are induced, unintentionally, but the shooter. Maybe 3-4% are caused by ammunition problems ... and maybe 1-2% are actually gun-related issues.
If you're shooting at a local range where there's an experienced instructor (familiar with shooting plastic-framed pistols), armorer or gunsmith, you might consider asking them to shoot your PPQ (using the same ammunition).
Also, if it were me, after I cleaned the pistol & magazines and properly lubricated the pistol (according to the owner manual), I'd try it again, focusing on consistency of grip technique over the range session.
It's also not uncommon for a bit of shooter fatigue to eventually occur after someone has been shooting 50-100+ rounds with a .40 S&W pistol, either, especially if you're not accustomed to doing so.
It might not hurt to try using some of the Winchester or Remington ammo, such as the USA or Express lines (meaning not the 100-rd value-type packs). Or some Speer.
Personally, I own and use an early production SW9940 (the licensed S&W version of the Walther P99 AS .40), and carried an issued one for a few years (firing many thousands of rounds through both, combined), and I know a friend who's fired more than 60K through his SW9940. (They all ran just fine once the revised Mec-Gar .40 magazines were received and used back in the early 2000's.)
I never particularly cared for the 155gr (and lighter) bullet weight loads in my 99's, but the 165gr & 180 loads I've used over the years have worked well for me.
Just my thoughts.