After living for a long time with filpped screen (used it primarly for bass in live situations), I finally managed to get it resolved myself.
Thanks to the troubleshooting done on this forum: https://www.eevblog.com/forum/repair/lcd-image-reversed-line6-helix/, I had a look to the connectors of the LCD screen towards the PCB. I removed the white glue on the connector, pulled out the (yellow) flex connector, gave it a good blow, inserted the connector and the screen is back to normal.