I've seen this reported before on another venue. It could be a bug in the latest release, but I can't test it because I don't have a second monitor.
Yes, I'm pretty sure it's a bug.
Do you remember where you saw it reported?
I'm not sure I fully understand your problem, but it may be related to the new "Start" workspace that is part of the Start screen experience. This workspace is special and will reset itself when it comes back up when the application is launched or all documents are closed. Once you open a document, you will be placed back into your "normal" workspace, whatever you've chosen that to be. In that workspace, you can still modify the placement of panels and it should stick.
So my recommendation is that you don't try to alter the Start workspace and make sure you've opened a document before you start modifying your custom workspace. You can also switch to your custom workspace using the workspace switcher in the upper right corner next to the search box when in the no document state and then modify it further.
Hope this helps.
It was reported on a prerelease forum, but no one else responded so it may have been an individual quirk.
I usually disable all this stuff that gets pushed on me in new versions - like welcome screen and CC Libraries panel - things that I'll probably never use.
The Start workspace seemed to be another of these annoyances, so I disabled it straight away.
I have now tried to enable it, and it makes the problem with panel jumping back to the second monitor go away - so that unsaved changes to my workspace are sticky.
I see that the Start workspace can be useful, and I might even use it, but I still think workspaces should work properly whether the Start workspace is enabled or not.