On a long underground rail journey, guests to my realm will have the mine cart suddenly stop part way along the journey while they're sitting in it. 

If I ride the cart, it goes start to finish perfectly fine (when platying local)
If someone joins my game, and uses the cart by themselves (leaving me at home base), then it gets stuck once they make it far enough away:

• The cart will refuse to move forward, but will move backwards.

• They can destroy the cart, then place it back on the track, and continue the journey just fine.

• Player can run around the tunnel just fine, it's only the cart that has an issue.

• Was working fine two weeks ago for everyone
• There is no lag or rendering issues going on, just the cart stops in that position, even if it's on a powered rail with redstone... it isn't budging until destroyed and replaced.

As a test, I tried moving my world to Realms, but this had the same issue however it occured earlier in the journey. 

Happy to be contacted if more detail or access to the game file is required. 
No mods or resource packs are applied.