When I loaded my world in this snapshot all the command block minecarts were gone. This bug doesn't seem to affect other minecart types. For some reason the commands still run even when the carts aren't there and you replace it with another minecart.
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Can confirm.
Just an empty cart dissappears? interesting 🙂
That "Interesting 🙂" makes me think there's something behind this...

Full command block carts and empty command block carts despawn.
Edit: Clarified cart type
No, only minecart command blocks.
Spawner Minecarts too!
Unable to restore the deleted ones sadly :'(

May the fix of this issue cause MC-85285 ?