UPDATE: Please read the comments I've left below on this post to get more information and or updates!
UPDATE #2: Video attached -> https://www.youtube.com/watch?v=XsZ-ticOUUo
Launch VRChat.
Use a tool like RTSS to monitor frametime and VRAM usage.
Open the Tracking & IK settings.
Toggle Selfie Expression on and off repeatedly. (Make sure to wait for Selfie Expression to fully load in first, it takes a second or two every time it's turned back on.)
Observe RAM and VRAM usage gradually increase each time you toggle it, instead of returning to normal.
VRAM & RAM usage gets even worse if you do the following.
Reproduction steps:
  1. Set selfie expression quality to Best Quality (higher quality will make the memory increase more noticeable).
  2. Then enable Selfie Expression, wait until it loads in fully, then turn it off, then turn it on again repeatedly.
As you repeat this part, VRAM usage goes up by a moderate amount each time, but does not go down.
  1. When VRAM steadies or does not increase, Turn Selfie Expression off, then walk around for a minute or two or stay at the same screen.
  2. Change Selfie Expression to any quality, and then repeat step 2.
What I saw: VRAM usage will keep going up and up until maxed out, then will fallback to RAM but doing steps 2, 3 and 4 repeatedly enough will cause RAM to be maxed out alongside VRAM. Higher-resolution webcams result in much larger memory increases per toggle (each frame/buffer/texture is bigger), so VRAM fills far faster at 1080p/1440p/4K and using higher Selfie Expression quality amplifies this effect.
Workaround: Re-joining the instance, joining a different instance or restarting VRChat fixes it and usage returns to normal level.