Bug maybe introduced with fix of MC-110474
to reproduce:
Build a track with activator-rail to eject player from minecart.
Then surround the activator rail with carpet or a block which is same as high, e.g. double layer of snow.
The activator rail will not eject the player on the carpet (which was possible pre snapshot 16w50a). He will be ejected on the rail from the direction where he is coming from.
Note: placing a minecart on an activator rail, then entering it will not reproduce the bug!
if you have any further questions, please ask.
Additional: this is on half slabs, too. I used them on several train stations.
Please do not mark unreleased versions as affected.
You don't have access to them yet.