Font rendering in OpenJDK is based on FreeType.
In OpenJDK 11 the quality is relatively bad on low-resolution displays (non UHD).
In OpenJDK 13 rendering is again better for special fonts, e.g. "DejaVu", but "Isabelle DejaVu" is unchanged.
Maybe there is some hidden configuration somewhere that can be tweaked to treat the Isabelle fonts like regular DejaVu?