That happens when the boiler depressurizes and the vacuum breaker doesn't open. A vacuum forms and pulls the gauge needle against the pin, changing its "zero" location. The vacuum breaker may be sticking due to scale build up, hence 1st-line's recommendation to descale. Descaling won't fix the gauge since the needle was permanently moved. If the gauge can be opened, you could reposition the needle, but often they're sealed. For what it's worth, the gauge is fully functional, you just have to subtract the new "zero point" from the actual reading.

This diagram and discussion from Can someone please explain false pressure? explains how the vacuum formed:



As for descaling, you need to "overfill" the steam boiler by disconnecting the wire to the water level sensor. The pump will then fill the boiler with descaler above the normal water level where scale forms. I haven't read Lelit's specific instructions, but descaling one espresso machine is basically the same as any other (search on how to descale an espresso machine). Assuming you're reasonably handy and follow basic safety rules like "unplug the espresso machine before removing case", you should also check the vacuum breaker is clean and functional.