I am having this same problem on Bedrock 1.21.21 as of today (17 August 2024).
I have rail that is within a single chunk (does not cross chunk boundary) and is repeatable with me in the chunk watching it happen (IE - I do not log out or leave the chunk in order to trigger the effect of the minecart stopping). Further - I tested riding the minecart and attempting to push forward (up arrow) to get past the affected block and the minecart would not move. I appear to have several blocks affected by this issue within the same chunk. At first I though this affect was because the rails were underwater, however I drained the area and recreated the problem without the water (IE on dry rail).
Today my carts started working! Not sure if anything was changed by Mojang. Very strange as I made no changes on my side.