For a period of time my son studied audio engineering and interned at a smaller (500ish capacity) concert venue in Cleveland, Ohio. The class was required to purchase and use the same headphones (sorry, don’t remember make/model) that measured extremely flat for their mastering projects. They didn’t sound particularly bad, but they sounded more dull than any other explanation I can come up with.
The biggest takeaway from this is that once recordings were mastered, they sounded better through the same flat headphones, but they sounded great through my both open and closed back “audiophile” headphones. Two different types of headphones because their uses were different, flat for engineering/mastering, and the “audiophile” ones for musical enjoyment. Pretty simple.