The bug
If you go to spectator mode and look underneath a minecart, you can see brownish textures which were used back in alpha, when minecarts were used for storage.
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Comments 18
Relates to MC-108636
Can confirm in 20w48a.
Still confirm 20w49a.
(NOTE: Notch is not removing this. which it meant to place item on minecart like, cave and cliffs mentions because how storage on minecart when you place gold and iron stack was it? If you want to get rid of it, delete the stacked texture right above the middle)
Can confirm in 20w51a.
Affects 1.17

Can confirm in 1.17.1.
Affects 21w39a

Can confirm in 21w40a.
[media]Fixed in 1.18 Pre-release 2
Can confirm.