Could be a internal spring button problem maybe a
quick mall spray of WD40 or other electronic safe lubricant
right inside the button might fix it,..
or could be on/off contact disintegration due to moisture, dirt, wear, etc, and has to be replaced as it is used so much on & off...at least it's still working...could be tome for service ......
or a good excuse to sell and upgrade...just sayin'

good luck
Paul.

PS you can surly order the button and attempt to open the bottom and replace it yourself probably only two wires etc..



https://www.youtube.com/results?search_query=replace+on+off+switch+yamaha+keyboard+