Issue
This bug seems to primarily affect some Poiyomi configurations where the 'Opaque' preset is reapplied to a material after another preset was applied.
Seems to have recently cropped up due to one of the recent mirror fixes in the last update as I and at least one other person on the Poiyomi Discord only noticed this on our avatars in the last two days.
Specifically this issue primarily affects materials that have their Alpha Destination Blend set to One as apposed to OneMinusSrcAlpha.
Attached IDs are of the same avatar using the exact same poiyomi material with the only difference between them being 'Alpha Destination Blend' being modified (See Attachment 2).
Avatar 1 (Has the Mirror Issue):
avtr_139d9b9a-03f7-44bc-8625-fc46bda95acc
Avatar 2 (Does not have the Issue):
avtr_ec659aeb-9359-4fe3-80a7-eb6878838fa9
Repro Steps:
  1. Change into an avatar w/ a shader where the 'Alpha Destination Blend' is set to one. Avatar 1 above is one such avatar.
  2. Open Face Cam.
  3. Change Face Cam Opacity to something less that 100%. In my testing mine was set to 50%. Lower opacity makes the issue more obvious
  4. Observe the transparency issues on the avatar.
Attachments:
Attachment 1 is screenshot is an example of the transparency issues. Personal Mirror is on the Left, Face Cam Mirror is on the Right with the issues.
Attachment 2 is a screenshot of the settings of 4 different materials with different Alpha Destination Blend settings. The ones with 'OneMinusSrcAlpha' do not have any issues in the FaceCam while those that are set to 'One' do have issues.