Losing a core when ally accepts that province in a peace deal?

[ForzaA]

You losed the core as you didn't had a conflict with the new owner in the last 50 years ... it may be your ally or not , it works the same

Indeed - it may not be the "perfect" solution, but the check is made only by looking at when you got the core, and when you last had a war with the current owner. Probably because new mechanics would be needed to track "when did I last fight for the province", while "last war" is stored between any pair of countries.

 

[ForzaA]

As it is, you could have owned the province for hundreds of years and have lost it only a few years, even months if you get complicated, and lose the core.

I don't think so (I can't say for sure though, losing core provinces and then have someone else take them before I can get back to the conqueror is rare indeed) since the "last owned" date would still prevent the core from being lost within 50 years.

In regards to your "core will be lost" - that would require the creation of such a function on a province basis.
But we're well off into speculative territory now - I don't *know* exactly how much it would cost in terms of extra coding and extra ingame maintenance cycles.