Material ambient occlusion does not work with the skylight set to movable. Setting the material to allow static lighting will fix the issue until the level is reloaded.
Found in 4.19 CL# 4033788, 4.20 CL# 4233996, 4.21 CL# 4234136
The ambient occlusion will not be present while the lights are set to movable but works with it set to stationary or static.
The ambient occlusion would show the same regardless of the mobility setting set in the skylight or directional light
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-62257 in the post.