In this rail configuration you can place a valid train stop although it does not show up on the visualization as being valid. Note that the station is valid as the lights are yellow and trains and pathfind to it. It can be placed from the map editor or via bots building the station from a ghost.

I ...

Possibly relevant to [0.17.50] Signal placement at rail divergence

It appears that the invalid signals be placed in 4 positions, dependent on the curved rail direction.
https://i.imgur.com/NRYLsQ9.png
Valid signals can be placed around it, as expected, but placing one on the same tile appears ...