I'm glad you brought this up. I noticed that and re-watched it a lot of times to figure out what happened. I'm glad I did, because I learned something!! The answer is quite tricky!
We're talking about the death of your last stock in that first match on Final Destination, correct?
Both down-B and side-B have start-up lag frames; the time before Sonic can possibly get an aura, ie. the ability to release a spindash. If you land on a platform during those start-up frames, your spindash jump data gets restored. But anytime after those start-up frames but during the spindash, landing on the ground does not restore the jump data. This includes SDR, shield-cancelling ASC, an aerial screech-stop from ASC or SD;
or, it turns out, even the ending lag frames from an empty ASC, whether or not it ever had a charge.
Watch the end of that last stock carefully. At one point you're hanging on the right ledge. You drop and quickly 2nd-jump towards the stage, and during the jump you do an empty ASC. A quick tap of down-B, it never even gets a charge. You got screwed by the worst possible timing; you just
barely land as Sonic's coming out of the ball, but this counts as having landed during the spindash, and so your spindash jump-data doesn't get restored.
Then you hang out on the ground for a while, fighting. But notice you don't get hit! If you had so much as jumped, walked off the edge of the stage, or been hit by an attack, then the spindash jump-data would have been restored. But you just walked around, and eventually use a grounded SC. Which still remembered that you can't jump out of a spindash.
It's pretty messed up -- if you had started that empty spincharge a teensy bit
later in that last jump towards the stage, it could have landed in the start-up lag frames, and you would've been fine. And with an empty never-gets-an-aura ASC, you can't really tell if you're safe when you land, aside from having good intuition about the timing window that separates the startup-lag from the ending-lag.
I hope that makes sense! I still can't believe your bad luck.... I'm getting myself into the habit of SHFF'ing after (or sometimes before) a spindash, just to be absolutely certain I can jump-cancel it.
Stupid Sakurai