No it is not. The production rule for function call is given as

function-call   ::= fn-name { expression }

That says, that the arguments of the call are expressions. 3 + 4 is an expression, so interpreting the input f 2 3 + 4 as equivalent to f 2 (3 + 4) is valid according to the grammar. That the grammar lists function-call (only) as an option for factor does not change that. The grammar is simply ambiguous. In more ways than this.

It is defined uniquely by the grammar; function calls have highest precedence.

It's probably implementation-dependent, and there do not seem to be tests with it currently.

Did you finish the kata? Which precedence order was correct?

I haven't completed the kata, but I would take f 2 3 + 4 as (f 2 3) + 4, in the same manner that 23+5 is taken as (23)+5