Paper2D sprites render only in left eye with Instanced Stereo enabled.
Also confirmed in Main CL 4329255
Sample Project with all variants of Paper2DSprite Parent Material attached.
Result: 4.19 All sprite materials except for TranlucentSpriteMaterials render only in left eye 4.20: All sprite materials render only in left eye
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-63553 in the post.