Does test_partial_reads fail for you when run separately? If so, it's for a different reason. Otherwise, there may have been text left in the input buffer from test_input that led to the failure, which is a separate problem that needs to be addressed via FlushConsoleInputBuffer.

> Until a fix is found, can we mark the tests as "known to fail" on a 
> specific Windows version? (So test_winconsoleio doesn't fail anymore.)

Maybe also split out the non-BMP case in test_input to a separate test_non_bmp_input case that's skipped or expected to fail if sys.getwindowsversion() >= (10, 0, 18362).

As to a fix, there's nothing we can do in Python. An issue can be opened at github.com/microsoft/terminal. I wouldn't expect ReadConsoleOutputW to be fixed. But they should be able to fix ReadConsoleW and ReadConsoleOutputCharacterW.