I thought the year ending March 2014 was their big loss, and it had a fair bit to do with storing the unsold Wii U consoles. I could very well be remembering wrong.
Maybe the year ending March 2013 was their first loss but 2014 also saw a loss?
I forgot they made a loss in 2014.
You said "first loss in decades" and the one that came to mind was FY3/2012 because that was due to the 3DS:
https://www.statista.com/statistics/216625/net-income-of-nintendo-since-2008/