When a minecart is travelling on rails through a nether portal in either the overworld or nether, minecarts no longer connect to the rail on the opposite side of the portal. (given the portals are the same size and facing the same direction in the overworld and nether and the rails connect to the same portal block.)
Note in the video, the rail (in both dimensions) connects to the center portal block of the 5x5 nether portal (both portals are 5x5). In 1.21.4, the Minecraft would continue traveling on the rail through the nether without issue. In 1.21.5, this behavior is broken.
Rail connections at the bottom of a portal still work although the minecart looses a lot of speed compared to 1.21.4
I used this feature to create multiple metro rail systems in my server and now they are all broken : (
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Likely related to the fix of MC-295395
This is because of another bug which the minecart turns the opposite direction to which it was coming in from.