For reference, it was fine in 4.24
In particular, jittering is severe when the camera is moving.
GaussianHalfRes has been removed in CL 14036168, which is likely to cause degradation issues if intended.
1. Create New Level in 4.27
2. Place a few thin boxes
3. Add PostProcessVolume
4. Enable 'High Quality Gaussian DoF on Mobile'
5. Set scale value to 2.0
6. Previewing Android 3.1
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-130682 in the post.