Playing on PS4. Happened for the fourth time now across multiple games.
Might be encountering a weird edge case.
You know the first city you found in the Ancient Era? For whatever reason, if I try to attach my old capital to my new capital to form a new megacity, the game crashes.
Could be I wanted a capital under a cooler name. Crash.
Could be I lost my first city early on, and retook later in the game. Will crash if I decide on megacity.
Even as an experiment personally razing my old capital to merge the outposts one by one to my new capital, once to merges the one territory my first city sat on, it will crash.