I bought my 650 new in December 2017. At 26k miles the rear-wheel bearings collapsed, damaging the wheel in the process, necessitating a new unit, supplied on Suzuki warranty. The scoot is now showing 46k miles and was in for service yesterday. The technician had to replace the rear-wheel bearings as he found lateral play while checking the bike over. So, OEM bearings failed at 26k miles, replacements worn to the point of needing changing at 46k miles. What's going on here? I have been riding since 1966 and have NEVER had to replace wheel bearings at less than 100k miles. (by the way, the fronts were changed just before the first failure of the rears)

Its usually water ingress that makes short work of them. Occasionally overload can happen through being pre-loaded because of bad assembly design tolerances. Except in the case of purposely designed taper roller assemblies there should never be pre-load on rolling element bearings. I have had to replace bearings on a Peugeot scooter at very low mileage because the design relied on the type of bearing which has a built in seal on the outward facing side and this just wasn't good enough.

Well, it's happened again! First lot of rear-wheel bearings failed at 26k, second set at 46k and now, in for sevice and first MOT, play found in rear and new bearings were put in. My guy uses sealed SKF bearings and, before fitting them, he removes the seals and adds extra grease. He's non-plussed as to why they are wearing so quickly but 9,000 miles is no mileage at all!
I never jet-wash the bike and, even though I am pretty hefty, the bike is not overloaded and I never carry a passenger.
Anybody have any suggestions?

Some clues ought to be visible from the old bearings. Are they rusty? Completely dried out of grease? Blued through overheating? Have the seals disintegrated? Do the outer or inner races show any signs that they have rotated in their housings or on the axle? As I wrote before any bearing will wear quickly if it is pre-loaded along the axis of rotation. This could be because the axle is over-torqued or because a critical spacer or shim has been left out or misplaced somewhere in the assembly. Putting extra grease into a pre-assembled bearing would be a waste of time but applying a little grease to seal lips and their mating surfaces does ensure that the lips are not damaged through initially running dry. On this occasion is the bearing play actually excessive? Some play may be felt even though it is within the design tolerances, it just depends on the design details and type of bearings used.

This was a continuous problem with a chap on the X9 owners site on a Burgman 400, turned out he was using the wrong grease. I think Roadster has covered the other points.

Seeing as most other things I can think of have been changed is the axal straight?