If min and max scale are the same in a LandscapeGrassType asset, scale will not be applied.
Confirmed in 4.25 MAIN @ CL 11318998
4.23 Sample project with basic landscape grass attached.
Result: Scale will be reflected if min and max are not the same. If min and max are the same, the scale will default to 1
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-88686 in the post.