When middle clicking a Spawner Minecart it fails to give back an item form for the Spawner Minecart and instead gives back a normal Minecart. Upon further inspection I found that an item version for Spawner Minecart doesn't exist despite it being an item like Minecart with a Chest and Minecart with a Command Block
Same with command blocks, Minecart with Command blocs, Barriers etc. but they all have item versions. Also this has a priority so the developers plan to fix this Christopher Lewis
Confirmed in 19w45a.