Sounds to me like a broken piston ring. These Burgman engines (like many engines) will run ok with a broken ring (usually top compression). I did a top end OH on a 2008 a few years back that had exactly the same issue you are reporting. Only thing, a broken ring, especially the top one will often make a small clicking as the engine runs...but it'll be down on power, crankcase pressure will be up, won't run nice. You could spend £8 on a phone usb endoscope/borescope camera from ebay...something like this:https://www.ebay.co.uk/p/2m-Endoscope-C ... 2599632400
It will let you take a look at your cylinder bore to see if there's any damage. A broken ring can cause scratching to the cylinder or sometimes no damage if you investigate and strip it early. The 2007-2009 399cc Burgmans had a different piston from the later 399cc engines because the piston use to flutter a bit at high rpm's. This can lead to piston ring breakages at higher mileages. Maybe, just maybe you have this issue.