1. Use a 10.1.0 installation, like our demo https://cms.demo.onehippo.com
2. Go in the channel manager, open any channel, press 'Edit' and then press "Edit menu" for a menu component
3. Click on a menu item that has an internal link to a document (so, don't pick the 'Home' menu item)
4. Try to change that internal link of the item to an hstpage, using the 'Select' button and the popup that comes up
What I'm noticing is that no matter if you choose "Documents" or "Pages (Sitemap items)" in the dropdown in this popup, the tree on the left always shows the Documents tree.