Parentheses are a little tricky in SPICE, and by extension, LTspice.

 

In many cases, parentheses are effectively ignored in SPICE netlist syntax.  Yet they are required in some cases.  For kicks, I tried a few simulations using the OP's user-defined function, and various combinations of matched and mismatched parentheses, curly braces, etc.  Some worked, others failed with the unknown "model"  error.  I did not see a clear-cut pattern so I concluded that trying to explain it might be an exercise in futility.  It appeared that matching parentheses was not a strict requirement in every single case, but sometimes it did make a difference.

 

Andy