Procedural Foliage Volumes can be resimulated by clicking the Resimulate button. But they can't be resimulated from Blueprints, even if there is a Simulate() function that can be executed.
Found in 4.20 CL# 4369336, 4.21 CL# 4753647, 4.22 CL# 6063286, and 4.23 CL# 6395561
The mesh will scale down but the procedural foliage will not update
The procedural foliage would update with the mesh upon using the Simulate() command
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-74466 in the post.