Also, I just realized that the levels in Idle saved but the price to upgrade them reset back to like 500.
Oh! By any chance, have you updated the game with the hotfix?
This update fixes issues and balances prices—now, they’re more affordable at lower levels but start increasing around level 29. Also, the max level for health and damage skills is now capped at 50, while the max level for attack speed is 15 (if I’m not mistaken).
If you run into any issues, just reset your progress. Yeah, I know—it’s annoying, but it’s the safest way to fix bugs. That said, updates shouldn’t corrupt your data, so it shouldn't be a problem.