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