This could very well be a bug, however, I'd like you to look into something:
There are items that may be causing you to get extra plague tokens such as the Deviant Package, this will cause you to gain extra plague every 24-hour reset.
You may have had some trades up somewhere to take plague maybe? Make sure that you check your trades, your abilities, and your items for any extra plague generation you may be making.
If you're still confused about the extra reply so that I can see if it can be looked into more!