Repro steps:

• Create a single player world, creative, allow cheats
• Give yourself a command block
• Set the command block to "repeat"
• Type this command:

  • tp @e[type=minecraft:falling_block] ~ ~1 ~

• Turn on "always active"
• Place a floating sand block nearby

Expected:

• The sand block constantly gets teleported in 0.5 blocks above the command block.

Actual:

• The sand block gets teleported above the command block.
• The sand block is deleted.
• An invisible entity? is created.
• The user glitches when touching it.

Code analysis:
Found in this comment by pr0cess.