First, a quick suggestion: if you haven't already, try switching to 'easy mode' in the gameplay settings.
Regarding the issue itself, it occurs exclusively on Android devices, usually those with low and medium-level power chipsets. Unfortunately, there's nothing I can do about it, just need to wait for a newer version that includes better optimization for mobile phones (I only recently discovered how to improve performance), or try to play on one of the most powerful devices from 2022-2024. The bar fills up quickly and comfortably when the game has 50-60 FPS on the high-end mobile devices, but when the frame rate drops below this range, issues arise and the bar resists more. In easy mode, this should be much less of a problem anyway.