And it causes the ovi to start the cell division that will create the next generation, form the "yellow body" and then effectively disconnect from the baby girl's body (obviously that cell division doesn't complete until a sperm combines with an ovi, hopefully multiple decades later)

So normally, this happens immediately after birth, together with dozens of other big changes.

Further those cells can’t compete out in a forest somewhere. Going multicellular means optimizing in ways counterproductive to survival on their own. But failing to cull off defective cells means you can’t have a functioning multicellular life form, thus making this a dead end.

So no in nature there isn’t some switch that means those cells get to survive indefinitely. The closest viable option is as an infection shared among very closely related organisms, but that’s not a stable option which means it’s really rare in nature.