If you are seeing robot avatars like in the image below:
Then the issue is being caused by shield mode and can be solved by doing the following.
Open your menu in VRC
Press Change Sheild Level
Set your sheild level to Normal or None based on your preferences.
You can also customize your shield level if you want to experiment.
If you are seeing Diamond Avatars like in the image below:
You need to disable avatar culling.
Open your VRC menu and tap the settings icon in the bottom right.
Scroll down until you see the header Avatar Culling
Disable both Hide Beyond and Show Nearest.
You can also experiment with the settings to find a medium for better performance