Let's say there was a live ball foul on the down that ended at the B5, penalty enforced, and clock started. Then there is a false start before the spike and the clock winds and expires before another snap. There would be 1 untimed down in this case even though the last foul wasn't during the last timed down. This is where I like the NCAA rule better and the clock has to expire during the last timed down when the foul occurs.
The LAST foul wasn't during the last timed down, but that's not the issue. The last timed down had a penalty that was enforced, so there will be an untimed down.
So let's timeline your scenario:
0:15 - play starts, gets to the B5, foul on the play, penalty enforced, clock now at 0:05, clock on RFP
0:05 - false start, so any spike doesn't matter, the play didn't happen. FS happened at 0:01
0:01 - RFP, clock runs out before snap
So yes, there was a penalty on the last timed down, so there will be another play. And I'll bet MANY crews miss this!
The rule doesn't say anything about the last foul, it only talks of the last timed down.