Hello,
I looked into this issue and found many similar reports in third‑party forums as well.
This behavior is commonly reported by Windows Insider users, because Insider builds receive features that are still under A/B testing. When the server‑side flag changes, Windows switches between the new and old Start menu design.
This is expected behavior and it is not caused by anything you did wrong.
Before we go further, could you confirm whether your device is enrolled in the Windows Insider Program (Dev, Canary, Beta, or Release Preview)?
The rollout of the new Start menu is still controlled by server‑side feature flags, and Insider devices are especially affected.
If you prefer the new design, the only option is to wait until the rollout is fully completed for your device. Once the feature is permanently assigned to your account/device, it will no longer revert.