A quick recap: During a falling block's first tick of existence (determined by its Time NBT value), it will check if it is inside of a matching real block. If so, it will delete the block. If not, it will delete itself. Command users have generally worked around this by setting Time to 1 or higher when summoning the falling block.

However, this is broken in 1.14. The entity will not delete itself, but it will delete client-side blocks. The blocks still exist on the server, and will reappear if the player relogs or attempts to place a block in them.

To test, run this command while standing on a stone block:

/summon minecraft:falling_block ~ ~-.01 ~ {BlockState:{Name:"minecraft:stone"},Time:600,NoGravity:1b,DropItem:0b}

(Note that Time is set to 600, causing the entity to immediately despawn. The bug still happens when Time is set to 1, but you'll end up with a number of falling blocks floating around your world.)

The block will appear to vanish, leaving you glitching up and down as the client and server are unable to agree on whether you are standing or falling.