The copy page functionality has a Destination dropdown, called Target URL in Hippo 10, that should be filled with the site map items from the target channel workspace, but it only shows the root URL.
The REST call behind the UI goes to /newpagemodel with a mountId parameter. If that
mountId is not there, which happens when just opening the channel, the site map items are found.
This is a result of
HSTTWO-3528 in which code was added to filter out site map items of 'inherited' site map items. However, the inherited flag is determined by matching the canonical path against a given prefix, which is the current editing preview configuration path call in MountResource#getNewPageModelRepresentation
Thus, when having a mountId parameter, the target mount sitemap items are flagged 'inherited' and will not show in the dropdown.
I've attached a local set up 11.2 project with the NL '/content' site map item moved to the workspace. When copying '/events2' from EN workspace to NL channel, '/content' should show in the dropdown.