Looks like Assign method fires for some properties and for some properties it's called even twice on the IF statement. I've tested just a few.

*SET STEP ON 

oForm = CREATEOBJECT("myForm")
oForm.Show(1)

DEFINE CLASS MyForm AS Form
    PROCEDURE Init
    this.AddProperty("nHeight",0)
    IF This.nHeight == 50
    ENDIF
	*IF This.Height == 50
    *ENDIF
    IF this.WindowState = 0
    ENDIF
    
     IF this.Visible 
     endif   
    ENDPROC
   

    PROCEDURE nHeight_Assign
        LPARAMETERS nHeight AS Integer
        MESSAGEBOX("Height_Assign called") 
    ENDPROC
    
    PROCEDURE height_Assign
        LPARAMETERS nHeight AS Integer
        MESSAGEBOX("height_Assign called") 
    ENDPROC
    
    PROCEDURE WindowState_Assign
        LPARAMETERS nState AS Integer
        MESSAGEBOX("WindowState_Assign called") 
    ENDPROC
    PROCEDURE Visible_Assign
        LPARAMETERS lVisible AS Logical
        MESSAGEBOX("Visible_Assign called") 
    ENDPROC
ENDDEFINE