I agree in the early stages, it's usually a big no no to mention any exes/ previous relationships. It usually means they're thinking of them and looking back (because feelings still there and not moved on) when they should be getting to know their new partner and looking ahead.
We never had long chats about our exes, just a few times they briefly came up in conversation. Only impression I ever got was that it was a happy time in her life that she enjoyed, loving Japan, and had an easy happy relationship. Never for a minute did I think anything more than that....