This may effect older versions but currently only tested on 21w07a

Typing the following command into the chat interface/command interface (Either one) will result in RED text after the key word 'all' when it is in-fact valid syntax. (And it does run as intended)

Here is an example:
/execute if blocks ~ ~ ~ ~ ~ ~ ~ ~ ~ all run say hi

Type the above command into the game and you will notice the syntax highlighting goes red for the words 'run say hi' (and there is also no Tab Complete once Space Bar is pressed After 'all' usually you can Tab Complete to any valid command which in this case should be any of the execute 2nd param, align, run, positioned, etc..)