Tab-completion does not work correctly for tags

The bug

Tab-completion and command syntax help does not work for tags. Once you typed the hash sign # it does not suggest anything for blocks, respectively an opening curly bracket ({) for items.

This probably affects all commands which support tags: /clear, /execute if, /fill ... replace, /clone ... filtered, ...

How to reproduce

Type or copy the following

/clear @s #

→ It directly suggests an opening curly bracket