Well... I've found the reason for the bug: Due to an oversight, holding down jump while landing may lock the player's ability to jump. It only happens if the player was landing with their "Dash" bar empty, and they were holding the jump button while landing. This lock-up gets removed when the in-game loop restarts (or the player restarts the game, which is quicker)
So, to prevent such lock-ups, players should ensure they're not holding the jump button as they land, or their dash meter is empty. A post-jam fix will address this bug (among many other :P :D).