New maps may inherit the default ways of previous saves and build ways that don't match the era.
Reproduction procedure:
(1) Start a new map in 2000.
(2) Build "ConcreteSteelCantileverRoad"
(3) Start a new map in 1750. (Don't end the game)
(4) Build "MasonryViaductRoad"
(5) You became a time traveler and brought asphalt roads to the 18th century. ;)
(https://i.imgur.com/XyRLsI4.png)
Is this due to the default selection of a waytype not resetting until manually done so? I've encountered the same issue, but being able to build track this way.