HandBrake is the industry standard for video conversion. It is free, open-source, and allows you to tweak every setting.

Use VLC for Android or MX Player with the "HW+" decoder enabled. iOS: Use Infuse (paid, but worth every penny). Infuse can load HD9 MP4s from a network drive (NAS) or local storage without converting them.

Before you hit play, confirm the following:

The "HD9" designation often appears in firmware metadata for GoPro HERO9 videos. Stack Overflow Optimal File Settings : For the best playback on other devices, use the H.264 (AVC) H.265 (HEVC) codec within an MP4 container. Resolution & Frame Rate : Standard HD is 1920x1080 at 30fps or 60fps. Transfer Method GoPro Quik app

This happens when you load an MP4 with variable frame rates. Return to your encoder and force Constant Frame Rate (CFR) .