Mobs are unable to pathfind while standing on a track.

This doesn't seem to be true 100% of the time. A baby and a regular zombie just pathed to me across the track. I was within three blocks of the track (whereas usually I'm 4 or 5+ blocks away and they get stuck), so it might be a distance issue.

Minecraft is really really strange there. Every time you test it you will get different results

The problem is where they can't do it while standing on track, not that they an't pathfind across it. Read each word in the issue.