I was going to do own 1001 provinces achievement. Took mission to conquer levant and cored all most of Mamluks lands. After annexing Syria lost all cores and got 300+% overextension and would have to recore all of it (i cant even release Mamluks because they still exsist).

I thought you have to core only same content colonies (provinces that have been actually colonized by colonists) or when linked up.

So make sure you allways have capital connection before doing enything with overseas.

I do not have art of war and it is just that or someone never did eny testing at all.