This isn't a glitch, but it's a bit illogical

[BobXP]

Did I spell that right?

Anyway, see the screenie attached... it should be self-explanetory

[eis_os]

There was already a post about it in the devel list,
maybe Marcin will fix it...

[Dinges]

it won't build roads in the angle where they don't have land...

[eis_os]

The use of "fix" wasn't right.

Here is the answer:

The problem is, I have to prevent people from joining roads on different
levels (I think you realize why). If I let them build road there (based
on, as you say, "already established slopes pattern"), then it would be
trivial to make a different-level joint -- just demolish one of those
tiles and build an 'inclined foundation' instead... The only simple and
sure way to prevent such dirty tricks is to use the underlying landscape
pattern without exceptions, even if it doesn't always 'feel' right.

I do realize this can seriously confuse people, but I just can't do it
any better, at least for now. I guess we'll have to say it many times:
foundations are not a replacement for real groundwork.

Of course, this will have to go into the manual, too.