auto is part of <'width'> already.

This is confusing because the auto value for flex-basis behaves differently than the auto value for width (or height). And what's worse, if you want the width (or height) to be 'automatically sized', you must use flex-basis: content.

Because the auto keyword, even though already part of the <'width'> syntax, behaves differently, I completely understand @p2227's mistake in missing it. What a pedagogical nightmare!

May I suggest setting auto to trigger the 'automatic sizing' behavior, as it is for <'width'>, and introduce a new keyword defer, which would mean "look at my width or height property"?

I'm afraid this suggestion comes a few years too late. Implementations have shipped, and web content depends on it. In addition to making the point that this would be a good idea, if you want this to happen, I think you need to present evidence that making this change would not result in many web-sites breaking, which I doubt is possible.