Well, I had some similar shifting issues. At the end, it was all due to chain slack and rear wheel alignment.
The problem was, every time I'd go to dealer, he'd ask me to sit on the bike and he'd adjust the chain slack. The bike would run and shift great for 20 miles or so but then the problem would return.
In the meantime, I changed front sprocket from 17T OEM (with rubber dumper) to 16T JT Front Sprocket (no rubber dumper). The shifting got worse. No metter how much you try with chain slack adjustment, the problem would come back. Then I changed 16T JT Front Sprocket to Suzuki 16T Front Sprocket (with rubber dumper). The dealer adjusted the chain and I asked him to respect the book - adjust on the side stand, 20-30mm slack. He adjusted to 25mm and my ride home was beautiful, shifting like dream. But, the next day the problem returned. Then I measured the slack on the side stand and it was 50mm. Adjusted it back myself to 25mm and checked rear wheel alignment. Alignment was out 1mm on one side. Aligned the wheel properly and went to ride. The shifting was smoother then ever, since I own the bike (almost two years and 18.000 km). It was almost a month ago and it still shifts like a butter.
Just my experience.
Had a chat with main mechanic at my dealer. He said that the slack on our bike should be adjusted by the book only. The front sprocket has a rubber dumper on both sides and the chain is slighly touching them. If you sit on the bike, the chain looks like it has no slack because of the tension provoked by the rubber. Many dealers adjust it with rider sitting on the bike and give it some minumum slack in that position. He said it was wrong because the rubber is fouling us. In my case, he said, the front sprocket was new and it needed few miles for the rubbers to bed in. So it was normal to adjust it after 20-30 miles again. But, he said, rear wheel alignment should have been checked earlier.
So, all good now.
2017 DL1000 White
2014 DL1000 Red (sold)
2012 DL650 White (sold)