Do you take two damage while boosting? I have died at 2 health sometimes when hitting a enemy mid-boost
I don't think there are Invincibility frames. I just did a run where I was only taking 1 damage while boosting. If it was only one enemy, then perhaps there was a glitch where the opponent didn't despawn before causing a second health of damage?
But it looks like intended behavior is no extra damage from boosting into a single enemy.