Extensions like Multi-Account Containers can cause this. Does it still happen in Troubleshoot Mode?

I do have Multi-Account Containers. After testing... yep... Opening a link that has a container associated with it is the culprit. Totally understandable, since sites in the Back (and Forward) 'history' would be available to the current page if left alone. The only way I can think of to fix that without compromising the 'security' of the container would be to take control of the 'Back' and 'Forward' functions, save those page links in the plugin, marked with whatever containers are involved, and have to close the current tab and open a new one for every back/forward operation - complicated, convoluted, and potentially a memory hog. Now that I know what it is and why it happens, I'll mark this as Solved.