So, I was testing the current TNT duplication method, trying to create it myself without following a setp-by-step guide, and pushed a minecart on a rail. To my surprise, it was popped off of the rail!
NOTE: The rail was on a slime block with a slime block in the direction of push.
I do however wonder if this may possibly be a feature in 1.15, as I have not tested it with the Honey Block yet.
What exactly is your issue? Can you provide a screenshot, or some steps to reproduce the issue?