The flight plan that would not load in this circumstance was:
KPVD/23 N0470F430 DCT PUT DCT NELIE Q75 GVE DCT MOL DCT KROA/06
And it seems like the Q75 Airway won’t load/does not work. Even if I enter it manually from NELIE it is empty/won’t apply.
EDIT: a few hours later, a totally different and unrelated flight plan that happened to include Q75 also failed to import (even after sim restart). Might be something about this airway or Q airways generally that aren’t working right.