Ok sounds like cylinder head issues; valve sealing, piston rings, pre chamber injector sealing?
Well Honda uses their own engineers, and most of them are young people who are very bright, but being bright sometimes cannot replace experience. This problem would have been detected by engineers experience with design these race engines.
Honda seems to be torturing McLaren by using Formula1 as R&D for their own development. I don't think they care about winning at all costs. It would benefit them to bring in a few consultants who have been in the game, ie European F1 engine designers.
I'm not bashing Honda, but it must be expected that things will go wrong with a ground up project; especially where there is little or no lessons learned from previous years.