I'm seeing the same problem.
1) Enter session
2) Unlock layer using the new Unlock button on the bottom right of the Layers tab.
3) Use the delete missing assets function
4) exit Surveyor
You'll see the program exits surveyor without asking for a save.
However, if you attempt to delete missing assets from a locked route/session, it will ask permission to unlock locked layers and then when you exit surveyor it WILL save.
hth
Anthony