Certain types of indoor lights have a pronounced flicker caused by the cycle of the electrical system. Sodium vapor lights have a flicker, as an example.
Why don’t we see the flicker? People have what’s known as persistence of vision. This essentially blurs the flicker so our brain never perceives it.
Cameras however do see it. The flicker in countries using 60 Hz electrical system flicker in multiples of 60. Those countries using 50 Hz systems flicker at multiples of 50. You proved this when you switched from 30 and 60 FPS recording to PAL, which records in multiples of 25 FPS.
What is the solution? Shoot in 24 FPS or PAL (25 FPS), change the light source or record the scene with a camera that has an adjustable frame rate to eliminate the flicker (some cameras have this automatically built in, some have manually variable rates and others have an off/on switch).