The ability to keep objects selected while jumping from one layout:detail to another is gone in R6.
Create 2 detail views with something showing in both. Activate one of the details, select the objects, and then double click on the other detail view, with the shift key down, to activate it. The objects de-select during this transition. I relied on this feature in previous versions… You can still get from model space to a pre-activated detail with selections intact.