From: El Paso, TX
The "Fill out with max ** Aircraft from Pool" Button will only fill the unit up to the current level of aircraft in the unit, not the official maximum. And after you click the button, not only does it NOT fill the unit up to the maximum, but now it greys out the "Draw one Aircraft from Pool" button right next to it, and further replacements are blocked for the next several turns. On the other, hand if you just use the "one at a time" button, it's possible to select the full complement of aircraft (albeit with a lot more "clicks")
The best example of this is visible at the start of any new game as the Japanese player. Select the Yokosuka Ku T-3 E8N2 Dave unit in Yokohama. It has 12 aircraft, but is capable of fielding 36. Clicking the "Fill out with max 24 Aircraft from Pool(70)" button only provides 12 new aircraft, after which the button disappears and the one next to it greys out. Reload the game and select the same unit. This time click the "Draw one Aircraft from Pool" button. You can do so 24 times until the unit maxes out at 36 aircraft (12 existing plus the 24 new ones).
Since the whole purpose of the "Max Fill" button is to reduce clicking, and since the "one at a time" button produces a different result, it seems pretty clear that this is a bug. Thanks for your time, attention, and support!