Why is that? Shouldn't the Key frames - the 'real' frames be unchangeable anchors?
No, I don't think so. Barring issues with "crooked" 24p framerates or NTSC framerates, the behavior to me is correct. The interpolation is averaged across multiple frames, forward and backward. What you want is to only interpolate forward, which I believe is a method RevisionFX' Twixtor offers.
Honestly, that doesn't jibe with me. Even if it looks 'backwards' ... take the B for instance in my example ... the previous frame generated would just get it's info from A making AB before B. But I digress - it is what it is but I SWEAR I've generated 60i fields from 30P without altering the Key frames in the past.
But something you said started me thinking. Twixtor. I don't have a few hundred bucks for Twixtor so that was out. But when I picked up Mark Christiansen's book he mentions in his discussion about frame blending that while After Effects doesn't come with Twixtor it DOES come Timewarp from RE:Vision so I looked into that...
and it worked.
There's a teeny tiny button called, "Build From One Image" that keeps the Key frames whole and makes AB frames in between as I would expect. (See! I'm not the only one who thinks it should work like this. .. ... they made it a FEATURE!)
So I got what I was looking for with Timewarp.
Thanks to Mylenium for the Twixtor hint and Mark for his great reference book.