This is I expect a PEBCAK, but: using Flight Plan 0.9.1 via CKAN, Node Manager 0.7.1, and no MNC install, I tried to do interplanetary transfers from Kerbin to Duna using next transfer window. A maneuver node was created, and the time until burn seemed correctish (given when the transfer window should have started), but when I'd warp to the burn time KSP2 would max warp and go past burn time and the time until burn would get something like 400-something days added to it. I'd eventually get to burn, but it'd put me in a Kerbol orbit that would take way longer than the transfer should have taken to get to a point where I could make another Hohlman transfer to intercept Duna.
I'm installing MNC now to see if it helps, but has anyone else seen this or know what's going on?