The bug
When typing a block or item tag (e.g. #minecraft:wool), the suggestions are shown without the leading hash symbol. When typing a function or entity tag, the suggestions are shown with the leading hash symbol.
How to reproduce
Type
/clear @s #
→ No leading hash in suggestions
[media]Type
/execute if block ~ ~ ~ #
→ No leading hash in suggestions
[media]
Type
/function #(with at least one datapack defining a function tag)
→ Leading hash in suggestions
[media]Type
/kill @e[type=#
→ Leading hash in suggestions
[media]
Confirmed in 1.16 Pre-release 3.