I used a Morty card and it increased my interest from 3% to 4%. It said the interest would last for a month. I collected my interest and now it says 3% again.

This is not a glitch or bug. The post HERE from Keith states that he reset it for everyone.

"Your interest will reset to 3% tomorrow. Turns out one of the servers wasn't properly updated, and those of you who were having issues were on that server. The 3% will go to 2% in a few days for those of you who do not have morty cards enabled. "

- curses if I had realised I wouldn't have wasted a Morty card -_-; thanks though

Yeah there have been quite a few people getting like 10% because it didn't reset like it was supposed to. But you are welcome! Locking this as resolved.