Basically you have forget to enable Screen Space Reflections and Refractions checkboxes, but it not necessary with Reflection Plane.
Currently Eevee couldn't reflect refracted surfaces.
For correct mirroring use Add > Light Probe > Reflection Plane:
Glass node setup with Camera crutch.
Blend and Shadow Mode could be Clip or Hashed.
EEVEE supports reflection by having a map that saves a screenshot of reflection.
You can add a reflection plane in front of your mirror. Also, the reflection setting in both the material settings and in the main render panel should be checked.
After a bit of digging when I ran into this issue again, I found out that for whatever reason, the vertex group for that particular bone didn't transfer to the mirrored mesh. As a result, the weights for the mirrored object were there, but the wrong bone was controlling it. In my case, I mirrored to the right side, and that part of the mesh was being ...