There is no fuel burn issue. The tip you’re telling people is not correct based on factual fuel burn values.
See my example below. 747-400 cruises at M.855. Let’s round to M.86 since we can’t do .005 increments. Trimmed out at 50% flaps up, I’m burning 22,300lbs/hr. Now if we look at the specs found online, you’ll see that this 22,300 number is actually within 1500lbs of what they’re saying online.
When I did your method, you’re doing 15,900lbs/hr. Now, sure its more efficient, but you’re flying in a nose down attitude (not realistic) and flaps at 10 degrees at cruise (not realistic). Not really glitching per say but you are burning less fuel than what is normal. So it could be considered. That being said, its likely that you are not step climbing appropriately, your aircraft is too heavy for the altitudes you’re going to, route of flight planned is not appropriate for winds, etc. All sorts of reasons that you could use Flaps 10 as a crutch to make up for erroneous planning.
Not saying your planning is bad, but if you have to use Flaps when the fuel burns are correct, there’s something wrong in the planning process.
So, I think before we make a statement and declare that the 747 has a “fuel burn” issue as you say with certainty, I would do the research first before making such an assumption. Fuel burn is spot on according to various sources.
Fly the aircraft with flaps up, normal configuration, plan appropriately. Never hurts to pack extra fuel.