Very unlikely that the lead is "too soft" as it's likely about pure lead swaged to shape. Which is what about every commercial manufacturer does for bare lead bullets. All "dead soft", as are hollow based wadcutters.

Could be a lube failure, as in "none", which would still look about the same as a normal "waxed" bullet.

Also could be the cases have sized down the bullet shank, so that the bullet is now too small for the barrel. Fairly likely, if the mfg. used the same tooling to load jacketed rounds, which can survive a fair amount of case "pinch" without being affected.

Shot a case lot (or two? Will have to check how much brass is about.) of PPU 158gr LRN .38 Spl. a couple of years back and my only real complaint was that accuracy was somewhat substandard for precision work. (can't remember if the Pythons got any, probably did at least a little.) Pulled a few bullets and did find them a little undersized, which was no surprise as there was no cannelure in the case behind the bullet, which is what was standard from Rem., Win., Fed., etc. back when 158gr lead .38 was probably the most common centerfire handgun round. (Mechanical bullet retention front and rear more than the common interference fit used with jacketed bullets.)

ETA: Oh, I remember now what was most annoying about the PPU(Prvi) .38 Specials! The wretched box dividers! Weak based posts that kept breaking and allowing the ammo to come adrift.

Their rifle ammo has been good to excellent, though.