in my current campaign as makedonia, something similar happened "naturally". I blitzed mainland greece and had it conquered in about 2 years, give or take a season. The Epeirotes were left with Taras, and I turned my attention to the North and East. 6 years on (or so) strategy led me to start growing into the West and it was then I discovered (I have fog on) that Capua, Arpi, Rhegion and Messana had all fallen to Epeiros, and they were beseiging Rome. The Romani hastily allied with me as I took Taras, and now the Epeiros, who should have left the game about 269 are finally getting their comeuppance.

But yeah, similar situation. Satisfying to see they are so tenacious.