Why does my FMP if condition not perform as I would expect it to do?

I have the following calculation in FMP to set a certain field. The funny thing is, that it is not performing as I would expect it to do.

If ( 
    MED_niveau = "Dir" or Left(MED_niveau; 3) = "Sen" or Right(MED_niveau; 2) = "PM"; 
    MED_sleutel_naam_medewerker_formeel_in_dienst ; 
    ""
)

We have a field called MED_niveau that holds the job level of an employee. This can be, amongst others, "Dir" in the case of the CEO, "Sen-Adv" or "Jun-Adv" for a senior or junior consultant and "Adv-PM" for a consultant with project management rights.

When I want to add the value of MED_sleutel_naam_medewerker_formeel_in_dienst to a field that contains the if-statement, it is not behaving as expected. An employee with MED_niveau of "Adv-PM" does not get assigned the value of MED_sleutel_naam_medewerker_formeel_in_dienst. It does work for the other clauses in the If statement.

I also tried to perform the same action by using nested If statements, but that also doesn't work.