Seiko Sumo running slow

I have a Sumo that I purchased new less than 2 years old and has always been 2-3 seconds fast a day. Recently it has been running very slow (10-15 seconds) per day and the only thing that I have done is let the watch run down when I wear other watches I own. I recently started to wear my Speedy everyday for the last two months so the Sumo sat in the watch box dormant and it's never been dropped or subjected to an impact. The watch is showing magnetism using the Lepsi app but I thought watches run fast not slow when magnetized so I'm wondering if if something else is up. Any thoughts would be appreciated..

I hand wind it at least 80 turns..

I've been wearing it for the last 2 weeks.. When I take the watch out and it's completely depleted it is not moving until I wind the watch in position one. The 6R15 movement can be manually wound as far as it's factory manual states..
https://storage.seikowatches.com/production/files/2018/03/20/143337762370/pdf/6R15D_E.pdf