When "r.EarlyZPassOnlyMaskedMaterials" is enabled, masked materials used with particles will flicker when close to the camera.
Found in 4.17 CL# 3658906, 4.18 CL# 3832480, 4.19 CL 4033788, 4.20 CL# 4369336, 4.21 Preview 1 CL# 4435822, and 4.22 CL# 4457235
The materail will flicker the closer to the camera is to the particle system
The material would not flicker
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-65185 in the post.