A 'linked' furnace-minecart & minecart(s) cannot stay linked on curved rails or rails that are angled downwards, they unlink and the minecart is flung behind or forward.

(video file is too large to attach)

Instructions for replication:

1. create rail line (with standard rails) that has a turn or 'dip' at any point and a block at each end of the rail to link minecarts.

2. link furnace minecart and minecart by coal-powering furnace-minecart into minecart and end of rail (that has a block at the end to stop it coming off the rails)

3. enter minecart and coal-power furnace-minecart

4. observe de-coupling when linked minecarts enter said turn in the rail