What I expected to happen was...:
I thought the minecarts would stay coupled

What actually happened was...:
The minecarts decoupled when going up incline or going down decline

Steps to Reproduce:

1. create incline/decline with any block
2. place rails on incline/decline
3. place minecart furnace on rails(flat) connected to the incline/decline
4. behind minecart furnace place regular minecart
5. place coal in minecart furnace so that it collides with the minecart thus coupling them
6. place coal in the minecart furnace to make it go up/down incline/decline

Edit:

after further inspection it seems like the minecarts hit boxes are a little messed up, as they phase through me sometimes when I'm pushing them up an incline, or when going fast enough, this also causes the minecarts to speed up for some reason. This could be the case for the decoupling of minecarts as well.