Edit: The reproduction won't always work. I had tries it out with multiple bridge types and it didn't work. A little later, I tries it out again and it worked just exactly the same way :/
What happened?
I was about to change a bridge in between elevated ways, so I destructed to old one and attempted to construct a new one. It won't work.
This does not seem to be limited to elevated ways but to any kind of bridge that does not start at a slope nor has a bridge ramp.
How to reproduce?
1. select the single tile raise "artificial slope" tool and raise the land by any height
2. Do the same a few tiles straight away in x or y direction.
3. attempt to build a bridge in between
- It simply won't work without raising a mesage.
4. Build two elevated ways somewhere at the same height.
5. attemt to build a bridge in between
- It won't work either.
Thank you for the report. Can I check whether this can be reproduced in Standard?
I can't even reliably reproduce this in extended yet... It definitely happens sometimes, but it needs further investigation before I can give any further details.