Edit: looking back at all your info (especially given that it only occurs with Admin rol), could be that the answer below if completely off.

Which version did you use before 5.19? Note that with the DX release (from 5.18 onward) changes were made to the layout system. This not only includes deprecation of the mx-layoutcontainers, but also changes to div overflows.

It's just a hunch, but if the previous version was non-DX related, perhaps this topic can help you out? Specifically the answer of Erik Heddema:

We deprecated an overflow property for dataview/listview/tabcontainers.

"Widgets that are able to scroll their content (e.g. data view and groups box) no longer define overflow if they should not show a scrollbar. Floating dom nodes that relied on the overflow of their parent to clear the float will have to clear their float explicitly."

Example: the class mx-dataview-content used to have overflow: auto but we removed that by default

combined with Michel de Bokx's comment:

Hi Erik, overriding some style elements within the BM 'overflow:auto' did the trick :-) Now I'm looking for which MX classes have been changed. I prefer the general class correction in stead of correcting all page/widget in the BM.