[Nasher]

Another possibility.

Has the bike had the little plastic carb breather containers removed?
Or the carb breather tubes mucked about with at all.

I had a similar issue as you describe on my M900 that had been modified by several different owners before it came into my hands.

I was tidying up some of the old mods to bring them up to my standard.
One of these was the carb breather tubes had been chopped off a few inches below the carbs as the breather boxes attached to the frame had been removed.
The Vacuum fuel tap had also been removed and replaced with a simple on-off version.

If I parked the bike up and forgot to turn the fuel tap off, fuel dripped out of the breathers down onto the top of the Horizontal cylinder. Luckily it took a while for the fuel to drip, long enough for the engine to have cooled down.

I found some of the correct size tube to replace the carb breathers and routed them down the frame to below the engine.
The bike ran OK at slow speeds, but once at a decent speed for more than a few seconds it ran out of fuel. I had to pull over and let the float bowls refill before it would restart.

I blamed a High/Low pressure area under the engine for affecting the float height.
I tried the longer hoses in three different positions on the bike, but it kept doing the same thing.
Eventually at the side of the road I cut the new pipes off in the same position as the previously ‘modified’ ones and it’s run fine ever since.

I plan to fit a vacuum tap, but for the last year or so have just remembered to turn the fuel tap off.

I’d also take a good look at all the vacuum hose to the fuel tap.

Nasher.