Damn
Working on it...
Can you attach project (before script use) and steps to reproduce (+tool settings)?
Ok, here logic going much complex.
So, there is "pos" (visual position on item) and "srcpos" (real position on audio source).
I can make limit, like "If final visual position much bigger/less then original 1.0x position - dont quantize marker". Hope it will works.
Note, all of this doesn`t work right if item with loop source.