I submitted a bug report and all for this issue to Unity. Unfortunately this is their response:
I contacted the developers about the issue you informed me. And they informed me that these issues should be fixed by the Pimax developers themself as this device is not in our supported devices list.
Also, they shared that at this moment we don’t have any plans for it.
Oh well, I never really had high hopes that they would fix it since they have plenty of other major issues to fix. Now they kick the ball back to Pimax when there is nothing Pimax can do about it.
So moving forward, all games made with Unity are still going to have this problem of objects popping in and out of your periphery. For devs, please use my fix here: https://github.com/koochyrat/SteamVRFrustumAdjust