I started a world in 1.14.x with seed "-562179930" to play on a local home server. Later it was upload to Realms where me and my son has been playing for a few months. It is now at version 1.15.2 (updated on Realms). We've been building mostly around spawn and one day my son found ~50 or so floating hopper minecarts around y=360 spread out over spawn when he was flying around with elytra.
I don't know when the minecarts appeared. Some of them have contents, just random stuff. I don't recognize any of the content and we don't have any hopper minecarts gone missing afaik.
I ended up teleporting around and killing entities within a certain radius to get rid of them.
Datapacks we use:
johalun@jm ~/.m/s/D/datapacks> ll
drwxrwxr-x - johalun johalun 20 Mar 15:03 anti creeper grief v1.0.0.zip
drwxrwxr-x - johalun johalun 20 Mar 15:03 anti enderman grief v1.0.0.zip
drwxrwxr-x - johalun johalun 20 Mar 15:03 anti ghast grief v1.0.0.zip
drwxrwxr-x - johalun johalun 20 Mar 15:03 coordinates hud v1.1.1.zip
drwxrwxr-x - johalun johalun 20 Mar 15:03 double shulker shells v1.0.3.zip
drwxrwxr-x - johalun johalun 20 Mar 15:03 more mob heads v2.1.0.zip
drwxrwxr-x - johalun johalun 20 Mar 15:03 multiplayer sleep v1.0.1.zip
drwxrwxr-x - johalun johalun 20 Mar 15:03 VanillaTweaks_c805239.zip
johalun@jm ~/.m/s/D/datapacks> ll VanillaTweaks_c805239.zip/data/
drwxrwxr-x - johalun johalun 20 Mar 15:03 rotten_flesh_to_leather
drwxrwxr-x - johalun johalun 20 Mar 15:03 universal_dyeing
drwxrwxr-x - johalun johalun 20 Mar 15:03 wool_to_string
Hmm, I just added a list of datapacks that we use and realize there's one missing. Initially we also had the "player grave" pack. I later removed that datapack and we had a lot of graves remaining in our world that I have to manually remove by killing their entities. Could it be that the player grave datapack put those hopper minecart over build limit to store the player's item at death? Sounds reasonable so this might not be a bug. I'll leave the ticket open but feel free to close this one if you agree that this was a datapack thing.