Looks like this can occur with motion blur off aswell, so likely it's a TAA motion vector issue.
Similar issue occurs in UE5 (see [Link Removed])
Download the licensee repro here - https://github.com/erebel55/RenderBug
Boot in 4.27, PIE, move around the level, the gun with blur quite intensely.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-116769 in the post.