help needed: I am unable to make progress on testing variable fonts

This is a follow up to https://github.com/googlei18n/fontmake/issues/167

From sources at https://github.com/googlei18n/noto-source/tree/master/src I managed to build the following variable files that have only weight axis:

The following newly built VF fonts have only single weight axis (axis="wght"); see https://github.com/googlei18n/noto-fonts/tree/master/alpha/from-pipeline/unhinted/variable-ttf/sans for the sans fonts and in ../serif you can find serif fonts:
NotoSansCherokee-VF.ttf
NotoSansLao-VF.ttf
NotoSansLaoUI-VF.ttf
NotoSansSymbols-VF.ttf
NotoSerifLao-VF.ttf
NotoSerifSinhala-VF.ttf

... sample ttx output for Cherokee

wght
100.0
400.0
900.0
256

When I run fontdiff tests (see https://github.com/googlei18n/noto-source/tree/master/test for the tests) I get the correct behavior. For example for Cherokee for test
```i

ꮓ̀
ꮓ̀
ꮓ̀
ꮓ̀
ꮓ̀
ꮓ̀
ꮓ̀
ꮓ̀
ꮓ̀

```
I get the following result in the PDF file created by fontdiff

The following newly built VF fonts have only both weight axis (axis="wght") and width axis (axis="wdth"); see https://github.com/googlei18n/noto-fonts/tree/master/alpha/from-pipeline/unhinted/variable-ttf/sans for the sans fonts and in ../serif you can find serif fonts:
NotoSans-Italic-VF.ttf
NotoSans-VF.ttf
NotoSansArabic-VF.ttf
NotoSansArabicUI-VF.ttf
NotoSansArmenian-VF.ttf
NotoSansDevanagari-VF.ttf
NotoSansDisplay-Italic-VF.ttf
NotoSansDisplay-VF.ttf
NotoSansGeorgian-VF.ttf
NotoSansHebrew-VF.ttf
NotoSansKhmer-VF.ttf
NotoSansKhmerUI-VF.ttf
NotoSansMyanmar-VF.ttf
NotoSansMyanmarUI-VF.ttf
NotoSansSinhala-VF.ttf
NotoSansSymbols2-Regular-VF.ttf
NotoSansTamil-VF.ttf
NotoSansThai-VF.ttf
NotoSansThaiUI-VF.ttf
NotoSerifArmenian-VF.ttf
NotoSerifEthiopic-VF.ttf
NotoSerifGeorgian-VF.ttf
NotoSerifHebrew-VF.ttf
NotoSerifKhmer-VF.ttf
NotoSerifTamil-VF.ttf
NotoSerifThai-VF.ttf

sample ttx output (Hebrew)
```i

wght
100.0
400.0
900.0
256

wdth
70.0
100.0
100.0
257

```
However, when I ran my fontdiff tests with these dual axis fonts I do not get the expected results.
For example for Hebrew Sans using https://github.com/googlei18n/noto-source/blob/master/test/Hebrew/specimen-he-weights-widths.html as a test. For the first line
```i

וַיִּפֶן
וַיִּפֶן
וַיִּפֶן
וַיִּפֶן
וַיִּפֶן
וַיִּפֶן
וַיִּפֶן
וַיִּפֶן
וַיִּפֶן

```
I get the following PDF output (note they all appear to be thin whereas I expected them to change weights)

Most likely it is not a fontmake problem: it might be a problem with my test HTML, might be a fontdiff problem, harfbuzz problem, pdf problem, etc... but I'm stumped and need help debugging this and fixing this (if this is an issue).