Scenario is similar to your fiddle demo with below deviations:
1) I've just 1 column with some options disabled.
2) Parent HoT has 4 frozen columns just before this column that has child HoT.
3) width is not set but autoColumnSize: true is set.
4) No header in child HoT
Looks like issue happens only in IE. In chrome I checked yesterday and it's working.
The issue is only while using mouse wheel to scroll in IE but surprisingly your demo works in IE though the scrollbar appears away from child HoT. For me scrollbar appears properly attached to HoT right side.