As others have mentioned, it is likely that the batch of tpe used on the sex doll has lost some of its ingredients and may cause premature aging. But I think you will notice that other areas, such as the armpits behind the knees, are also worn out. It seems to be limited to private use, so it reminds me that the chemicals you use to clean up are the culprits.
Vaseline / mineral oil itself is a poor promoter of bacterial growth, so just mixing light soap in water immediately after use and washing it occasionally is enough to prevent the risk of mold. I think people will continue to read the horror stories here, seeing past topics, freaking out, and then sometimes getting tidy. Just be thorough, but not necessarily heavy on the use of chemicals and hot water.
Just to give you some ideas on where to start fixing the work that has been done. You need to get a sample / spare tpe from the piper doll manufacturer, apparently having the same skin tone. You want to use tools. Therefore, as Arthur mentioned, it is a hot air rework station. I will also get a stainless steel sculpting kit like this to help you add filler tpe sex dolls and smooth the area.