From what I gather from the manual and changelog, this LED indicates whether you have selected a „positive values only“ waveform for the LFO (instead of applying the default workaround by showing waveform LED + Random LED).
It does not indicate that the value of the LFO is currently positive.
To use Studio on newer Ubuntu Desktops you need to install the old libwebkit2gtk-4.0.so.37.
To do so create a sources.list file for apt containing the following line:
deb http://gb.archive.ubuntu.com/ubuntu jammy main
And install the lib.
sudo apt update
sudo apt install libwebkit2gtk-4.0-dev
After this delete the sources.list file.
More infos on https://www.weigu.lu/music/midibox_hp_2x2/index.html