Images are not shown after some levels

Hi thunkers.

I develop a quiz game for students and the game has 100 levels.

I performed testing on apk file by playing continuously from level 1 to level 100.

However, at level 36, the app became slower and at level 38, all images (back button, hint icon, clear icon, button background) for the quiz were broken / not shown.

I had to relaunch the app, and everything back to normal again. However again, at level 75, the images were broken once again.

Anyone might have idea why this happen?

**the screenshots are below

How the quiz screen should be:

The images are broken:

Show your relevant blocks.