Screenshots rendering too quickly, resulting in artifacts.

Since 4.9, taking highresshots can result in what appears to look like an image with an overlay of black dots. I often use a value of 6 for my highresshots and never had an issue before 4.9. Sometimes it’ll work, then sometimes it’ll complete the process far too quickly, and result in an image like the one seen here:

Apologies if this issue has been pointed out before, I cannot seem to find any queries that are like mine.

Thanks for any suggestions!