His sprite changes, but he keeps effectively gliding.
Letting the jump button go makes him drop as usual, upon landing he rides the snowboard normally.
Making him drop as soon as he touches the showboard should fix it. Making him fall in ball form would be preferred, because otherwise the board would appear from nowhere in Knuckles' hands, that would be visually jarring.
In fact, there was already code in place for Tails when he touches a board while flying. Replicating the same logic for Knuckles while gliding seems to be enough.
His sprite changes, but he keeps effectively gliding. Letting the jump button go makes him drop as usual, upon landing he rides the snowboard normally.
Making him drop as soon as he touches the showboard should fix it. Making him fall in ball form would be preferred, because otherwise the board would appear from nowhere in Knuckles' hands, that would be visually jarring.
Reported by bennascar as part of another issue.