It has been broken since a couple of versions, I didn’t exactly notice when. At least since 3089.
When I start with Glyphs sources, I also use the Glyphs hinting and export.
If you feel brave, you can manually correct the cvar table with ttx. It will look something like this:
<cvar>
<version major="1" minor="0"/>
<tuple>
<coord axis="wght" min="0.0" value="0.0" max="1.0"/>
<coord axis="ital" min="0.0" value="0.0" max="1.0"/>
<delta cvt="0" value="0"/>
<delta cvt="1" value="0"/>
<delta cvt="2" value="640"/>
<delta cvt="3" value="654"/>
<delta cvt="4" value="576"/>
<delta cvt="5" value="592"/>
<delta cvt="6" value="448"/>
<delta cvt="7" value="462"/>
<delta cvt="8" value="0"/>
<delta cvt="9" value="-20"/>
<delta cvt="10" value="-192"/>
<delta cvt="11" value="-206"/>
<delta cvt="12" value="320"/>
<delta cvt="13" value="300"/>
<delta cvt="14" value="0"/>
<delta cvt="15" value="64"/>
<delta cvt="16" value="72"/>
<delta cvt="17" value="64"/>
<delta cvt="18" value="0"/>
</tuple>
<tuple>
<coord axis="wght" value="1.0"/>
<coord axis="ital" min="0.0" value="0.0" max="1.0"/>
<delta cvt="0" value="0"/>
<delta cvt="1" value="0"/>
<delta cvt="2" value="0"/>
<delta cvt="3" value="0"/>
<delta cvt="4" value="0"/>
<delta cvt="5" value="0"/>
<delta cvt="6" value="0"/>
<delta cvt="7" value="3"/>
<delta cvt="8" value="0"/>
<delta cvt="9" value="0"/>
<delta cvt="10" value="0"/>
<delta cvt="11" value="0"/>
<delta cvt="12" value="0"/>
<delta cvt="13" value="0"/>
<delta cvt="14" value="0"/>
<delta cvt="15" value="16"/>
<delta cvt="16" value="40"/>
<delta cvt="17" value="16"/>
<delta cvt="18" value="0"/>
</tuple>
<!-- ... more tuples follow, based on the amount of your masters -->
The first “tuple” element is wrong, it must be deleted including its deltas. Then recompile the font with ttx …