There are numerous reasons on how your Rarity bar can revert to level 1. In summary failure to log-in for a day and/or to trigger the feature properly will result to this issue or event. If you want to know more about the mechanics of the Daily Login Rewards and reasons on how your Rarity Bar can reset or revert please refer to the article Here.
Multiple Device Users
Aside from the linked article above, there is a big possibility the your Rarity Bar will reset to one (1) if you are a multiple device user playing in a single account. Since you are doing what we do not recommend anyone to do, Daily Logins is only meant to be monitored by the system in a single device. With this situation, different account problem may arise and this issue is one of those. We believe that it is your responsibility to take care of your account and refrain from doing or taking actions which the game is not meant to be doing or played, the Brave Frontier team or gumi will not be held responsible for such actions.