Fresh 3.5.1 install.Took a UA config that was backed up in 3.5.0, and
imported it without the Security information.

Trying to assign a container to (a shared) Page Permissions, we can search
for container, select containers and move them to the right "current

But - When we press SAVE, the added container is no longer listed under
"current assignments".

No errors or messages can be seen on the server's console / log when we do

There is no problem in adding Users or Groups to Page permissions.