It's a small ball of TPE that closes the end of the vagina in TPE dolls. Not all dolls have this design but a lot do, including many of the name brands. Some have taken steps to correct it while others have not. Seems like almost all generic dolls have this issue.
The ball comes loose after (sometimes before it's even delivered) which leaves a hole into the interior of the doll. The metal skeleton gets wet from playtime or cleaning and begins to rust.
Some of the more professional tinkerers have found ways to fix it. I don't trust myself to not destroy my doll and take extra precautions like using an air-mattress pump then aquarium air pump for at least 1-hour for drying. Also I insert new tampons as far as possible after each session. I have a scope camera and so far, no rust but it's a painstaking process.
The doll I got here has this issue. I have 3 others that do not so I barely use the one I bought here anymore. The price was fair enough at the time that I'm not really upset about it but I would be if I paid what they're asking now.
That has a ton of info on it.
You can make a TPE plug if you have a portion/chunk of TPE material, same as the dolls are made of. Had a doll that failed (still pissed about that LDD, take heed) and started to rust. Cut her up, saved large portions of the TPE flesh to make plugs which you will basically have to carve into a plug shape yourself, then insert into the vaginal passage all the way to the back using whatever you got that's long enough and will push the plug all the way to the back of the vagina without it turning or stopping. Super hard to do but possible (you shouldn't have too, just saying) Get your shit together LDD! Basically I trashed a very expensive doll because of a design flaw, saved a good bit of TPE material, and took the rest of her to the dump.