Steps to reproduce:
Use the option "Show Bookmarks" in the "Bookmarks" menu, observe the visible sidebar, then use the option "Hide Bookmarks Sidebar" in the "View" menu, observe that the sidebar is no longer visible. Now open up a new tab and observe that the sidebar is again visible. Then notice that doing these steps in reverse ("View" menu option and then "Bookmarks" menu option) has the proper outcome of no visible sidebar on the new tab page.
All options that show/hide the sidebar should provide the same outcome of visibility.
Orion, OS version; hardware type:
Orion Version 0.99.113.2.1-rc (WebKit 614.1.12), macOS Monterey 12.4 Beta, 2017 iMac
Note: I believe this is the actual cause behind issue #1485