Is the point character vulnerable post-Alpha Counter?

To the best of my understanding, the point character is supposed to immediately become invulnerable as soon as the Alpha Counter screenflash is seen. And, up until today, that's the experience I've had with it.

However, playing with Dormoxx today, I had his Painwheel at low-health in a blockstring when he attempted to Alpha Counter out. The screenflash went off just as usual, and Big Band came in with his assist. But immediately as Big Band came in, Painwheel was hit and killed before being able to leave. I couldn't replicate a character getting hit post-counter in training mode, so I'm guessing what happened is that the Alpha Counter was used on an attack that was blocked, but also caused chipdeath? Is that supposed to be possible?