1. Summary / Title: At war with liege and their vassal breaks away causes issues
2. Description: 

I'm at war with opponent yellow who is liege of purple. Purple is their vassal.

I'm attacking both, reducing warscore of yellow and not purple (so far so good).

Halfway through the war purple breaks free while I have one of their Cities occupied. I'm now at peace with purple but still occupy their city, and still at war with yellow.

When the war is over, I vassalize yellow.

I'm still occupying the purple city.

I'm passively gaining and reducing warscore with purple as if I'm at war but I'm actually at peace.

I cannot release the city, nor claim it, nor build anything in it.

I get messages that I'm close to a war victory.

1. Steps to Reproduce: 

1. Start a war with an opponent that has a vassal
2. Occupy a city of the vassal
3. Make the vassal break free
4. Win the war and vassalize the opponent

1. Reproducibility: 

Don't know if it's reproducible

1. Expected Behavior: 

Not sure what desired behaviour is, but either when the vassal breaks free, have them be at war with you, or be at peace and give the city back but keep the warscore toward their previous liege

1. Notes
2. Attachments: 

I'm occupying the city:

Still at peace:

Getting 'Close to war victory' messages even though I'm at peace

Save game:

auto-save 229.ctr

On a side note, when a diplomatic demand is granted, please state which one it is. It's difficult to find if they converted to my religion or if they paid for trespassing for example.

UPDATE:

Going to war with the purple and then vassalizing them as well resolved it, the City went back to purple territory.

I had the same issue LINK