ZDoom

Fixed. I did not use the patch though.

Ok, that's acceptable. I so far refused to address this issue because it would have meant making the teleport search logic prone to mapping errors.

But you guessed one thing wrong. This map does not work because some teleports have tags and tids but the destination only has tids, not to mention that tags never worked on mapspots. So the proper compatibility fix needs to be a bit different. It shouldn't do a second search for mapspots but instead only ignore the tags.

Somewhere on Doomworld, in a thread that was absolutely not about ZDoom or its bugs, someone was complaining about Happy Time Circus being broken. I looked into it; the issue is that it uses map spots instead of teleport destinations. There's even one that's very malformed.

Here's a patch that lets HTC work again, but through a hidden compatibility option so that it will not allow other modders to make the same mistakes.