If the navigation simulates what would happen if we follow links to SPA#pos1, SPA#pos2, etc so that if I do two clicks within the SPA, and then hit Back three times I'm back to whatever link I followed to get to the SPA, I guess it's OK and follows user expectations. But if it is used as an excuse to trap the user in the SPA unless they kill the tab, not OK.

>> So that in single-page applications, it can work intuitively instead of always taking you all the way out of the app.

Just implement an additional back button on the SPA. This is actually not confusing and is done in some places. Navigation buttons within an SPA are common enough.