Agree 100% with CPH-Flyer. I flew SEA-NRT-BKK in early April, back on the same route in late April. Mask usage was pretty much universal both ways.

Culturally, Japanese attitudes just seem to be, "We all follow every rule, always, with no exceptions -- and expect you to also". Not just about masking. "By the book" is how they roll.

My flights were wonderful.