Just ever so slightly unrelated as I can't comment on a 250r (it arrives tomorrow - woohoo!) but I had a similar experience at much lower speeds and I never really got to the bottom of it...
Was a Marin Mountvision full suspension bike and whenever I got beyond 15mph and took my hands off the bars (which I often liked to do after a really hard ride) the handlebars started to wobble and would increase the rate of wobble to the point of crapping my pants and I'd rapidly grab the bars again.
This had nothing to do with my riding ability. I can easily go round bends without my hands on the bars and rode so often hands free whilst chilling out that I can do quick little flicks and change direction without a problem on roads or slightly uneven terrain. Done it for years on bikes of varying quality, £50 to £2000 and never had a problem till my most expensive bike lol - go figure.
I took it to 5 different specialists including the manufacturers cause I was really narked at this - a £2k mountain bike should be able to do what a £50 bike can do - go in a straight line lol.......
and all of them told me my bike was 100% ok, absolutely nothing wrong with it. They threw all sorts of crap as to why it might be doing it; some blamed my riding, the weight from the reflector on the wheel, the weight from the valve and in the end all I got from the manufacturer was that I shouldn't be riding it with no hands in the first place.
So I sold it and have a bike for half the price that goes in a straight line again.
Now it could be an induced feeling due to lack of confidence in the bike which I had ever since the first handlebar wobble, but I can swear that the wheels were working against each other when going round the corners. I could feel it, the bike never felt right going round corners and on a perfectly flat road could not go above 15mph without crazy handlebar shake if I took my hands off.
And for me - a bike that cant go in a straight line on multiple flat roads and feels very odd around corners has an alignment problem, no matter how slight.
Maybe full suspension configurations can accentuate a normally acceptable slight misalignment to a problematic handlebar wobble
I would suggest you experiment with your bike on long bends at varying speeds and really concentrate as to how the wheels feel. You've been riding a long time, you'll know what I mean if the wheels don't feel like they are going round a corner together.
I would imagine that you would only need a very slight misalignment to cause some wobble at 50mph.
Or as Marin told me.....stop riding hands free lol
