When used in "say" commands, selectors will still act as selectors and output the player names. "@f" is not a selector, and yet when it is used in a say command, it returns "That player cannot be found". This does not occur with other non-selectors, such as @d.
Linked issues
Comments 6
Tails, that's actually not correct. If you type "/say @s", the command will output a literal "@s". The /say argument doesn't require a player parameter, so it isn't looking for a player named "@f".
It seems the game interprets "@f" as a selector, even though the only valid selectors are "@a", "@r", and "@p". All other @<letter> strings are just interpreted as literal strings, making the behavior with "@f" inconsistent. This should probably be re-opened, as the behavior doesn't actually make any sense.
@f is an invalid argument, so the command block assumes it's a playername -> could not be found.