I just threw in a ZoomExtents in case that was the problem and it doesn't make any difference.
NinjaEdit: In two locations (within the bounds of the drawing frame) this
Dim acTypValAr() As TypedValue = New TypedValue() {New TypedValue(DxfCode.Start, "*")}
Dim pnt1, pnt2, pnt3, pnt4 As Point3d
pnt1 = tmppntcoll.Item(0)
pnt2 = tmppntcoll.Item(1)
Dim selFilter As New SelectionFilter(acTypValAr)
pmtSelRes = Active.Editor.SelectCrossingWindow(pnt1, pnt2, selFilter)
correctly selects the Mtext object.
In the other locations even when Zoomed to Extents it doesn't find the MText entity.