I believe this is a regression in the train stop placement logic. In previous versions of 0.15, it was not possible (unless my memory is even worse than I thought) to place a train stop in the middle of a "switch" like this:
That is the furthest into the curve I can place it. There is also one slot to the east/right (from screenshot perspective) that is also "too close" to the curve.
I'm not sure if Squeak Through affects rail collision boxes, but I did have it enabled at the time when I observed this behavior. (Also Beautiful Bridge Railway and Naked Rails, but hovering over the rail pieces in the screenshot shows no yellow mod name text, so I don't think any mods are changing the vanilla rails used here.)
[0.15.39] Train stop is placeable "on" curved rail
Re: [0.15.39] Train stop is placeable "on" curved rail
Hmm, I thought Squeak Through would be the culprit of the issue, but I was able to reproduce it in vanilla 0.15.39 (and 0.16)
Re: [0.15.39] Train stop is placeable "on" curved rail
Ooh, kind of neat to have found a bug in 0.16 before release.