Update: I see this was fixed in 2.8.6 - thank you!
For some reason Windows Sandbox isn't showing up in the menu for me (Windows 1903, StartIsBack++ 2.8.5).
Other changes get picked up immediately (e.g. if I copy and paste an existing shortcut in the directory). I thought it might be a permissions issue, but the values under the Security tab for the Windows Sandbox shortcut don't look any different to normal.