When a Minecart Furnace (with fuel) hit an other Minecart (except Minecart Furnace) the Minecart will have a huge boost. The boost is 3 time longer then a Powered Rail Boost! Here is a video
PS: In the video there is a Powered Rail but it also work if the Minecart Furnace start on a normal one
Thank you for your report!
We're actually already tracking this issue in MC-89221, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.