I fixed the issue in svn rev 9643.
For the previous dev. build, I fixed some weird behavior which looked like a bug, but it turned out that some code was working only thanks to that weird behavior, and hence it broke.
Here is the difference between working and broken maps:
If .map file has "name" spawnarg set on the worldspawn, then the map loading was broken.
If .map file does not have "name" spawnarg on worldspawn, then it map loaded fine.
Of course, some existing FMs (e.g. Window of Opportunity) have "name" spawnarg, so that was a serious issue.
The next dev build should work fine in all cases.