I was wrong to say that the SMW physics always prevent you from proceeding, but it seems to occur more often than it doesn't for me. Unknown factors seem to affect where Mario will stop on the platform. You'll first notice something is wrong if Mario stops in the air one tile to the right of where the higher ledge of the platform ends. He will run and fall off the lower ledge before he can jump. Sometimes the score tally will start before the death is detected and the cutscene will proceed as normal with Mario under the lava. In other cases, I've seen Mario stop on the very edge of the upper ledge or gravity take effect before he stops. The cutscene will proceeded as normal in both of these.
Seed: fa963; SMB1 World 8-4 replaces World 6-Fortress 1. I had to make a savestate and resume on a version of the hack without the SMW physics.