When add "Related Products" to another Product, if the Product list has multiple pages, and Related Products are selected from more than 1 page, only Product(s) selected on the last used page are actually added to the Related Products grid.
Products spread across 3 pages, select 2 from Page 1, 1 from Page 2, and 2 from Page 3 - only the 2 from Page 3 get added.
Tested many times and this is the consistent behavior. I would expect all selected items to be added at the same time - currently this involves doing a separate "Add" for each page containing Products that should be selected. Not a huge issue, but ...