IF NOT MyMethod(foo) THEN does not work (A warning)

I know it's hard to beleive. I have spent all day debugging some code and have finnaly found the culprit.  This is the offending code:

/* ==========================================================================
   LoadPhoneLine
   ==========================================================================*/ 

  METHOD PUBLIC LOGICAL LoadPhoneLine(p_PhoneLineGUID AS CHAR):
    DEF BUFFER PhoneLine FOR PhoneLine.
    
    FIND PhoneLine WHERE PhoneLine.PhoneLineGUID EQ p_PhoneLineGUID NO-LOCK NO-ERROR.
                     
    IF NOT AVAIL PhoneLine THEN 
    DO:
      ASSIGN THIS-OBJECT:PhoneLineGUID = ""
             THIS-OBJECT:PhoneNumber   = "".
    END.
    
    ELSE
    DO:                      
      ASSIGN THIS-OBJECT:PhoneLineGUID = p_PhoneLineGUID
             THIS-OBJECT:PhoneNumber   = PhoneLine.PhoneNumber.
    END.
    
    RETURN AVAIL PhoneLine.
  END METHOD.
METHOD PUBLIC CHAR GetPhoneLineConfig (p_PhoneLineGUID AS CHAR):
     DEF BUFFER PhoneLine     FOR PhoneLine.
     
     IF NOT THIS-OBJECT:LoadPhoneLine(p_PhoneLineGUID) THEN .
     DO:
      MessageManager:LogError(SUBSTITUTE("No Phoneline [&1] Exists",p_PhoneLineGUID)).
      RETURN "".
     END.
     RETURN "stuff".     
END METHOD.

The method GetPhoneLineConfig always returns "" . However, if I change it to read

METHOD PUBLIC CHAR GetPhoneLineConfig (p_PhoneLineGUID AS CHAR):
     DEF BUFFER PhoneLine     FOR PhoneLine.
     DEF VAR Check AS LOGICAL NO-UNDO.
     ASSIGN Check = THIS-OBJECT:LoadPhoneLine(p_PhoneLineGUID).
     
     IF NOT Check THEN 
     DO:
      MessageManager:LogError(SUBSTITUTE("No Phoneline [&1] Exists",p_PhoneLineGUID)).
      RETURN "".
     END.
     RETURN "stuff".          
 

then "stuff" is returned.

As mentioned, I spent most of the afternoon finding this piece of code. I checked. I double-checked. However, no demo code I produced gave the same problem.

Can you spot the problem ?

(Hint. It's empty)