I watched your video.im sorry to say this but my tiger did the same thing before it died. It was a year in captivity also and died healthy.i did document it and the behaviors on video but it's saved on my laptop which is getting repaired.i will post it ASAP.my tiger started the shakes or twitching yours is doing and died fat and healthy about a week later.it was due to stress I think from my flower always attacking it during feedings.
Is your flower getting picked on? Ppl don't realize that flower/tigers do not handle any sort of harassment easy like other more common kept species.they are fragile in that sense.my flower is in perfect condition right now so I would definately rule out seasonal change. But I would not rule out that something has changed inside your tank? Basic things trigger this in fragile species.my flower is 2 years captive now.after I lost my tiger healthy I've run a straight reverse osmosis drip in that tank with great results.ive even seen breeding behavior out of my male.
I would recommend keeping things stable ,this is key to keeping the fragile species of rays. Try a divider if possible and do it in a manor it will not stress the ray at all. I'm glad you got through it last year ,it's probably due to the fact you'rs is so long term captive now it's toughened up.
Does it still eat?