Transnistria has been in a stalemate since before stopped calling itself a Soviet Socialist Republic; it wasn't (re)invaded later.

Historically, Transnistria in USSR was used as military foothold, to capture Moldova, pretending that this is just a conflict or a civil war. Russian Empire/USSR/Russia used that trick many many times. For example, Ukraine was captured by USSR using "Kharkiv People Republic" controlled by Moscow. Today, Russia tries to capture Ukraine again using "Lugansk People Republic" and "Donetsk People Republic".

Transistria is at border of Ukraine and Moldova, owned by both, with roughly equal populations, but both Moldovans and large part of Ukrainians were at same side. For example, leader of separatists was captured by Moldovan police in Ukraine capital city. Why "conflict" happen if both native nations were at same side?