Test Case:
Create the following configuration of track:

 N
W E
 S

RXXR
RXXR
^XXV

R is regular rail, ^ is teleporter rail facing north, V is teleporter rail facing south, and X is nothing. Put a minecart on the north end of the east rail and push it south onto the teleporter.

Expected Behavior:
I expect the minecart to teleport to the west rail, moving north.

Actual Behavior:
The minecart teleports to the west rail, but then immediately teleports back to the east rail and rolls south off the end of the track.

Demonstration:

Versions:
Minecraft 1.12
Forge 14.21.1.2420
Forgelin 1.5.1
ShadowMC 3.8.0
Extra Rails 1.3.0

Other Notes:
If the east teleporter rail faces north instead of south, it still doesn't work right, but the problem is different: in that case, the minecart will endlessly teleport back and forth between the two teleporters. Also, if you build the exact same setup but rotated 180 degrees, then it works as expected.