It's not a bug. The "Automatic Prizes" is a feature that can change the prizes that are awarded based on participation. Let's say you typically host tournaments of 10 to 30 players. If 10 or fewer players show up, you configure prizes of 50%, 30%, and 20%. But if 11 to 25 players show up, you configure prizes of 46%, 27%, 17%, and 10%. If more than 25 players show up, you configure prizes of 42%, 25%, 15%, 10%, and 8%.
You could simply wait until you're about to start the tournament to configure the prizes, since at that point you'll know for sure how many players there will be. But if you do that you can't show your players what the prizes will be until the last minute. It would be nicer if (a) the software could configure the prizes for you and (b) if it could do it on the fly, as players buy-in, so the prizes displayed are accurate (for the current players bought-in). This is the Automatic Prizes feature. Enable it and it will do this for you, based on a file that describes the various prize "levels".
When you have Automatic Prizes enabled, buying players into the tournament (or removing them from the tournament) can cause the prizes to change automatically. This is the default behavior. It can also be configured to be changed based on other criteria, but usually it's just "number of players in the tournament".
If this is not what you want you should disable it. On the Prizes tab, press the Automatic Prizes button.
FYI, it's not enabled by default. So it had to have been enabled, either by someone manually enabling it, or by loading a tournament that has it enabled already.