Problems have still occurred with Gen IIs and 500 series Minis.
I'm no machinist by any stretch of the imagination, but since there's a majority opinion Tapcos are generally GTG, and the mag's dimensions are fairly consistent, the only remaining variable is receiver dimensions.
Realizing Ruger and most manufacturers of repeating firearms were ramping up production over the last few years, it's likely more than a few guns have hit the shelves at the upper and lower limits of parts tolerances.
Those tolerances allow for the addition of an outside part (say a magazine) made to a very narrow specification. Ruger knows what those dimensions are, but aren't making those numbers known.
I would guess Tapco reverse engineered their magazine, and used several receivers to test fit.
Could be Tapco is updating the molds based on additional samples from the 500 series to address the issue.