Reproduction steps:
Stand in water in creative mode
Place lily pads around yourself by holding down place button and dragging mouse around.
Outcome:
Lily pad item flickers on hotbar, and sometimes vanishes entirely
Expected:
Placement to leave item in hotbar like any other block
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Can confirm. Also, this is not a desync issue, the lilypad is gone from the inventory completely. Scratch that, forgot that creative inventory is client-side. Was able to reproduce in Survival. This is a desync issue.
Might be related to MC-9591

Lily pad placement logic was entirely replaced in 20w22a, so I doubt it. More likely, MC-9591 is now fixed.
Confirmed for 1.16 Prerelease 2

Relates to MC-172898