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It looks like there might be a contradiction in the documentation of import in the language reference. In the loading subsection https://docs.python.org/dev/reference/import.html#loading, first bulleted list of the section, it specifies that if a loader fails, it must remove only the failing module from sys.modules, while other side-effect imports by loader will stay in sys.modules.
In the next subsection https://docs.python.org/dev/reference/import.html#loaders, second bulleted list (last bullet), the phrasing is confusing, and possibly contradictory. I believe that it meant to reiterate what was said above (see my previous paragraph).
Attached is a potential patch, but if anyone finds that I am interpreting this wrong, I'll make adjustments accordingly.

New changeset f473063318c3 by Brett Cannon in branch 'default':
Issue #22242: Try to make some import-related loader details clearer.
https://hg.python.org/cpython/rev/f473063318c3 

So the second point isn't contradictory, just more thorough (and had a mad mix of singular/plural wording). Loaders could add more than one module if they chose to. The key point is that when a load fails, only the modules that failed and that the loader itself was directly involved with loading should be removed from sys.modules.
I tried to fix the plurality of the words in the second part to help clear things up.