This is  not just a 2020 issues.  I have had this same problems getting the Hierarchical view to stick in 2019, 2020 and 2021 versions.

Sometimes I stumble into something (never really know what) and it will stick for a time.  Seems that when another user does certain it all goes back flat view.

This is extremely irritating and agree with other posts.  We pay enough for the software that this should be fixed after years and not working.