I think this trio looks nice together, with their similar appearance. One is mine, one is @Aethelred's, and one is his now but formerly mine. The Antoninus Pius is in my present collection. The Hadrian also used to be mine. (Prior provenance to the Boston Museum of Fine arts and @AncientJoe.) It has since gone on to live with @Aethelred, and I have lamented that more than once, but I do get visitation rights once a year when I get up to North Carolina. Boy, that stern portrait is stunning, ain't it? The Marcus Aurelius belongs to @Aethelred and has resided in his collection for decades, I believe. He wanted to acquire my Antoninus Pius on this recent visit, and I'll admit it looks nice with the other two, but I managed to resist, for once. It's enough that he wrested that Hadrian from my grasp already, right?
My LCS has (or had) a Antonius Pius for sale very similar to yours, @lordmarcovan however it was missing a chunk out of it. it was quite larger than I anticipated! Cool stuff!
Could be the flan was just split along the edge as a result of a hard strike? Depends on how much of a "missing chunk" you're referring to. I gather that some flan cracks resulted from striking, and are not all that uncommon. Then again, if it's actually broken, that's a different thing.
There is a lot going on when hammer hits the die. Sometimes these things happen even to Gordian III which has a crack to reckon with. Most people would not touch this coin with a ten foot pole but I love it. 'Defective' happens when a coin leaves the mint and has a hard life. The mint defined what was acceptable. Someone at the mint probably said 'good enough for government work' and let this one go.
I find I don't mind flan cracks all that much, in the relative scheme of things. Mind you, the example you shared there might be a tad egregious for me to consider it for my own collection, but on the other hand, it IS interesting, and at risk of using a cliché, that coin certsinly has lots of "character". Better a "defect" like that than some big scratch, or gouge, or breakage, or surface damage, which happened after it left the mint. After all, that crack just evokes the very creation of the coin, right? And I think I see your point. It's not really so "defective" at all, in this case.