I had the same happen to France in one of my games. After i took about 30-40% of France and got their war exhaustion to 100% they became stuck in a constant series of revolutions. First Communists, then Monarchists, then Jacobin etc. And this was my goal all along,.
I Would say it's pretty realistic. Russia did loose ww1 pretty bad in OTL, and got a Revolution. But after they formed the USSR, there was still Reactionary rebels(White Russians) they had to fight, leading to years of war.
But this is probably why Victoria 2 should have the civil war system for Rebels that was discussed on this forum.
Like if Communists rebels form the USSR, the Reactionary form the White Russia, Anarchists form a Anarchist nation etc.
Then the first years you will have fighting all over the place, as there are Communists, Reachionary and Anarchists etc in the same provinces. But this is also how a civil war starts in Real life. So then after a while the strongest Rebel Group will orginise itself and start to occupy territory. Then the Civil war could be like a normal war, that will go on until one side wins, where the other GPs could interveen if they had a Casus Belli. Types of Casus Belli could be, same form of goverment etc, like if countries with a Monarchy fights to prevent Communists overtrowing a Monarchy. This to prevent all GPs from joinin in.