Volumetric lightmap holes appear every fifth build.
Found in 4.19 CL# 4033788, 4.20 CL# 4302132, and 4.21 CL# 4348893
Results:
The volumetric lightmap will have areas missing on the fifth build.
Expected:
No gaps would be in the lightmap.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-63964 in the post.