It’s a “feature”, not a bug. Typical Blizzard.
Considering the many many conditional modifiers that can either affect damage through a specific situation (e.g. damage against burning, damage while you have a barrier, damage while fortified, vulnerable damage, crowd control damage, damage against close targets, etc.), the D4 devs thought best to not include these in the overall Attack power value as they’re, well, conditional.
In D3, as far as I know, the Attack Power (or DPS, whatever it was called) took almost everything into account as pretty much everything took your base damage, and applied whatever skill modifier on top of it. It was easier to see if your “upgrade” was actually an upgrade or not.
In D4, damage calculation is convoluted, and it appears that even the devs don’t entirely know what’s going on sometimes with the many multipliers. As far as I know, there is even a “Loading screen” tooltip (or there used to be one) that stated that Attack Power was a very rough estimate that did depict a very light “picture” of your overall damage as it didn’t take everything in consideration. As such, they know and they state that their own Attack Power stat is kind of useless.
Honestly, considering how hard it is to actually know what damage you’re doing, even by testing it, I would say they should simply remove the Attack Power stat/tooltip once you hit lvl 60. The Attack Power value is kind of useful when you first start your character and level up to 60 to easily gauge whether a new weapon hits harder or not. Past that, it’s useless and should be removed as there are many modifiers from either Paragon, Aspects or passive stacking that you can’t see through the Attack Power stat, and these multiplicative modifiers are where your true character potential comes from, not the base attack power. Here again, working as intended, but very misleading.
When it comes to playing the game and pushing through harder content, my first advice would be to stop paying attention to the Attack Power stat and to learn about and start stacking multiplicative damage modifiers.