Please try to reproduce your observations with the code below. What version of VFP are you using?

UPDATE: modified the code. I do see this in VFP8.

SET STEP ON
ox = CREATEOBJECT("_form")
ox.show(1)

DEFINE CLASS _form as form

	top = 0
	left = 0
	height = 200
	width = 200

	ADD object _cntText as _Container with ;
		top = 10, ;
		left = 10, ;
		visible = .t., ;
		width = 20, ;
		height = 20

	ADD object cmdLeft as commandbutton with ;
		left = 20, ;
		top = 80, ;
		height = 24, ;
		width = 70, ;
		caption = 'Test', ;
		visible = .t.

	PROCEDURE cmdLeft.Click
		ACTIVATE SCREEN
		? THISFORM._cntText.Left
	ENDPROC

ENDDEFINE
DEFINE CLASS _Container AS Container

	PROTECTED PROCEDURE Left_Assign
		LPARAMETERS tnVal
		DEBUGOUT [assign fired - ] + PROGRAM(PROGRAM(-1) -1)
		IF VARTYPE(tnVal) <> [N]
			RETURN .f.
		ENDIF
		THIS.Left = tnVal
	ENDPROC

ENDDEFINE