Not sure if this is a bug, but it bugged me:
I ran away from the core systems because everyone hated me but Tri-Tachyon.
Eventually Hegemony overran and destroyed every other faction.
With just me (7 systems) and Hegemony (30ish systems) remaining, I started getting ready for the final stand against the massive Hegemony.
One of those steps was to give away one of my small planets to revive Tri-Tachyon and ally them.
Immediately after flying away from my newly-revived, fledgling ally, I left the system to go face the Hegemony horde, and then:
https://cdn.discordapp.com/attachments/176402489849937920/620103009925332992/unknown.pngA little anticlimactic...
Edit: Looks like TT went and allied HG right after I left, causing the 'diplomacy victory'. I'm pretty sure TT was wiped out by HG, so that's kinda weird.
Oh well, I guess my dramatic final stand was averted by everyone deciding to hold hands. I'll just say that TT came up with some new diplomacy tech.
Edit 2: In the 2 in-game months that I've continued playing after receiving this "victory screen", random diplomatic events have caused my relationship with TT to go from +52 to -11, and my relationship with HG to go from -something to +45. lol...