Well... i try the editor in Q5.
I can't go too long...
But, i think this is one thing you must correct, and I suspect you're doing...

If I create a room called A, and a room called B.
I can tell the program in A: Player starts game in this room
But, also, I can tell the program in B: Player starts game in this room.

That, looks like is incorrect...

Yes, I agree this doesn't seem correct!

I'm not sure what the best way to fix it would be though, because that checkbox simply sets "start=true" on that room, and you could in theory set that on multiple rooms if you wanted (but the "game start" script in the library would only move the player to the last room with "start=true").