wght/opsz axis mapping interference

When generating a variable font with mapped optical size axis I am experiencing some problems.

It looks and behaves alright in fontview 0.3.1, but eg. on Axispraxis (using Chrome) there is a visible jump whenever the slider is moved from the wght minimum with opsz placed anywhere not minimum. The axes’ defaults are defined at the minimum, but I also saw similar behavior when I put the defaults at the maximum. I attached a sample file to better illustrate the problem.

Is there any way I can prevent this?

[WghtOpszTest.zip](https://github.com/googlefonts/fontmake/files/3898155/WghtOpszTest.zip)