Yellow fuel hoses pathing around heat shields no longer allow fuel transfers.
(I do NOT think this is a duplicate of #25694, because I did the "time warp" workaround several times and it still persists. Even after reloading the scene, as the very first fuel transfer I attempt (so bug #25694 isn't triggered yet), this still won't work.)
On hard difficulty setting, the game does not allow fuel to transfer through a heat shield. But there is supposed to be a way to design a craft to have an umbilical connection around the heat shield, using the yellow fuel hose parts to provide a secondary path for the fuel to follow. I used to do this all the time. As of KSP 1.10, it stopped working. Fuel hoses going around the heat shield no longer provide a path that fuel transfer will use (even though ignited engines WILL use them to burn fuel through, fuel transfer won't use them anymore.)