Reporting in from the no-break-up route:
- When convincing Sayra for peace but betraying the rebels to Gisette, the same weird thing happens: "A plan", normal not-faking breakfast, sick Sayra & Clarmont, then Sayra becomes queen and marries Jarrod. I did see the line about the treaty including a Sayra x Jarrod marriage this time (it's possible it was there in the break-up playthrough too and I just missed it after all), and got a new line about Jarrod realising this means he and I must break up and being pissed about it. Everything after that continues normally for the Sayra x Jarrod ending, with Jarrod remembering he was pissed at me for the break up and my PC going home unmarried.
- When successfully negotiating for peace, new weirdness: I got okay-but-not-perfect peace in Revaire, with Sayra becoming queen and Gisette starting a business venture... then got the Jarrod bad end where I marry him and became the crown princess/future queen of Revaire. So no bigamy this time, but bi-royalty...?
(On the other hand, I'm guessing there's only the one ending to successfully ("successfully") becoming queen of Revaire in normal non-buggy supporting-the-royals route, and it's not a bug that I got a bad end even though I had hit all the Jarrod reform flags up to and including the "aghast" pre-MM one... but that is such an almost hilariously brutal ending, love it 😅 The Matchmaker DID warn us there was no changing him, huh! Now I understand why there are no Jarrod route bonuses.)
Also, more of a continuity weirdness than a real bug I suppose, but when not breaking up with Jarrod, I get introduced to the Revaire ambassador as his future bride twice: first at the start of W6, when Ana and Lyon intervene, without seeing the actual convo with the Marchioness; then a second time more explicitly during the ball. The Marchioness does act surprised seeing me.
And for a normal bug, during the last breakfast visit from the girl friends, I got a Penny sprite hanging over behind Cordelia during all of Cordelia's scene, but I don't remember which combination of Jarrod & Revaire route variations this happened in, sorry!
-ETA- Seems to be tied to the "partings are inevitable" answer!
Still, this went relatively smoothly, the game doesn't just full-out break and crash, and the Sayra x Jarrod solution does make sense when supporting both sides, it's just weird that Gisette comes up with that on her own instead of just going murder murder as usual 😅