In my ongoing game, when I attack the vassal of my ennemy it triggers grievances and increase the war support of the ongoing war. Seems like a bug, grievances are not supposed to supply war support during war, right?

Here is a screenshot :

In addition, the turn before this one I captured a city from the vassal, with multiple territory attached, the war support jumped by 30!

EDIT: If a dev want to have a look, here is a save : https://dropmefiles.com/j315s

Just captured another city (4 territories) from his vassal, and boom 8 grievances triggered (4 'oppressing my people' + 4 'oppressing my faithful') :

Something weird happening here, it's going to be quite a long war. ^_^

Here is a save if a dev wants to take a look : https://dropmefiles.com/j315s