Your theory is sound. .380 (Ooops... 9mm. Duh) pocket pistols such as the P938 are designed for full power loads. What you would carry in the gun... when you carry the gun.
Semi autos are a balanced system... drop out of, what I will call the range of balance, and you have issues, either with with function, or gun damage.
The other theory is the gun is new and tight. After a more extensive break-in, along with a clean and lube, it may function just fine with the lighter loads.
Cheers,
C