It took me a while to figure out exactly what's going on with promotions, but I think it actually must have been impossible to promote with negative mana to begin with. There are some Leagues where you can see that seemingly people would have promoted with negative mana (e.g., this one), but didn't because the bar for promotion was just moved right below the lowest person with positive mana. Although the display is glitched out, so it's hard to know for sure what happened.
@Eliza used a code to run through all the Leagues data to see if anyone was in a higher division the season after their mana was negative and found that there was only one case where this happened. But that case was because of @Catnee moving from the Silicon League (for bots) to Platinum, not because of promotion.