ligatures in variable font Fraunces

[olsak]

I installed Fraunces free variable font from here: https://github.com/undercasetype/Fraunces/releases but the ligatures fi, ff don't work properly. I renamed the file name (in order to remove [...] from its name) and I tried

\input luaotfload.sty

\font\fa=Fraunces-Regular
\font\fb=Fraunces-Utype-Varfont:axis={wght=400}
\font\fc=Fraunces-Utype-Varfont:-liga;axis={wght=400}

\fa aha fialka

\fb aha fialka

\fc aha fialka

\bye

The +liga font feature is default in luaotfload. The static font \fa gives correct result, but the variable font \fb withwght different than 800 prints bad ligatures. When ligatures are disabled in \fc then the result is correct.

I don't know if this is issue of the luaotfload or of the font. But static font works properly and it is generated from variable source, isn't it.