Slitherine

The Celticii proposed a peace treaty in which one of the clauses, I as Rome would relinquish Insubria as one of my objectives, which I was currently occupying, and 100 metal. I counter-proposed, removing the Insubria request. They accepted, and I still lost Insubria. I'm not sure how this process is supposed to work?

You have two things going on.

#1 You occupy a region.

#2 That region is listed as one of your objectives.

From what you've posted, your enemy asked you to remove that region from your objectives list. You refused.

You did not demand that the region be ceded to you, so it was not ceded to you.

You sent this proposal and it was accepted.

So the region is still on your objectives list and you no longer occupy it.

in your counter proposal you should have picked, cede province, the province. All provinces you conquer during the war turn white in the diplomacy tab when you enter diplomacy with your enemy, and then if you want to keep the provinces you captured, in your peace request you must demand that they cede all the provinces.

Its like Paradox games diplomacy, you dont automatically keep everything you conquered during the war, but instead have to demand the specific regions you want to keep.

You did not demand that the region be ceded to you, so it was not ceded to you.

Yes. I completely forgot that step. Thanks.