The bug-

When Minecraft is placed in a water-logged Minecart rail, when player is standing right beside it the Minecart Disappears.

How to recreate-

1. Get a Minecraft, rail, some block (preferred to be transparent) and water in a bucket.

2. place surrounding blocks and put water inside of it.

3.place your rail inside the water to turn it water-logged.

4.place you minecart and push it to the edge of the block.

5. start walking on top of the transparent block and see when you'll get close to it the minecart disappears.

6.if you  want you can use the new lightning rod block chain on any other block that acts the same to see that the Minecart will disappear only in the water-logged area. 

7.I have uploaded a video of how to do that on reddit https://www.reddit.com/r/Minecraft/comments/l1rwl4/i_think_i_found_a_minecart_bag_in_the_new_snapshot/?utm_source=share&utm_medium=web2x&context=3