I get error messages when mirroring "Shapekeys" (see yellow field in the bottom right corner of the first two screenshots below) but I can't see any visible irregularity:
1) "Mirror Shape Key" ("1646 vertices mirrored, 2 failed")
2) "Mirror Shape Key (Topology)" ("1058 vertices mirrored, 1446 failed")
With the "Mouth Corner Out Shapekeys" for the left and the right side active there's no visible difference:
