iphone 12 video problem

On top of the Variable Framerate thing, I'm going to guess you're in the HEVC codec, which is one of the worst codecs for video editing. Also, I'm not really sure what sort of "HDR" is really coming from an iPhone 12, but generally speaking HDR is not a simple thing in a video editing workflow. Basically, you have three major factors contributing to difficult playback: VFR, HEVC, and HDR.

Those clips use a particularly nasty version of the VFR ... variable frame rate ... format. You have a constant 'frame' rate (sample rate, really) for the audio, but the video frame rate changes constantly as the phone "sees a need" for frames.

For example, you could have the phone set to 30fps. But in MediaInfo when you check the file properties, it might say that it's VFR, and frame rate varies from 26.92 to 31.1 frames per second through the clip. But Premiere has to have a specific number of frames per second, every second. So it needs to create additional frames for much of the clip in various spots, and in other bits, cut frames.

Some VFR media Premiere can work with not too shabbily. My Samsung phone is often ok. But even then, it's easier and typically better quality to get the VFR clips converted to CFR in the free utililty app HandBrake then replace any 'original' clips used in the project with the converted ones.

In Handbrake, be sure to go to the video tab, and click the CFR circle and set a specific number in the frame-rate box. Then set compression to "near placebo".

Neil