OK...this is crazy strange.
I don't think the core has changed since I last tested (the day before the question about composite blending was posed)...and it was DEFINITELY adaptive, because the difference is visually quite obvious.
I just retested, and it indeed appears to (now?) be full-screen blending...sort of.
With blending on or off, watch the title screen as the waterfalls pass behind the logo on Sonic 1. While they are partially obscured, they are unblended...and once they are fully visible, they 'snap' to being blended...even if blending is disabled.
That's freaking bizarre.