Not the OP, but we have seen this issue before and for us it was caused by the same texture being used in both a decal material as well as a normal static mesh material within the same level.
The streaming information for the static mesh takes precedence over the decal’s texture streaming through the fallback path for “unknown” textures, which means the mips for that texture will always be based on the distance to the static mesh only.
So if the static mesh using the texture is far away, but the decal is close, the higher mips are not streamed in correctly.