This is correct. Ironically, the bug is the fact that on some comps (not on mine), you can actually queue more than 1 Titan at a time. It's when you've built the foundry, and haven't researched the 4th Titan research yet.
I know this because I tried to take advantage of the bug and build more than one Titan. It didn't work.
But, yes, the one Titan limit is part of the game. We can't have too many gargantuan machines of flying death around, can't we?