Okay, a little bit more testing proved that the every three I was encountering earlier *was* just how the RNG happened to play out. Was seeing more of a spread during the second pass of testing.
Also, the "flickering" I'd noticed earlier seems to specifically be a case of a noticeable-yet-short delay between images in the sequence. I'm not familiar at all with Game Maker (on the coding end) so I can't say for certain that's what it is, but it's at least a consistent enough to seem noteworthy.
Viewing post in v0.6e Pregnancy and Girl/Girl action (w/animation) comments
It seems to be a loading issue from my tests. The groping and kissing lose the flicker after the first round of each, but getting pregnant and giving birth multiple times a session doesn't seem to help much. Which likely means it is a cache issue.
I am asking people if they know solutions, though it might just be rpgmaker jank.