This is actually true for both files and untitled files. While this issue has been there for files already without hot-exit, it is a bit more obvious now for untitled files at least.
Preserve language picked for untitled text documents (fixes #16803)
Actually fix was easy enough for untitled. Opened #17115 to track for files.
I am seeing this with 1.9.1 on windows 7
Needs #207 for empty workspaces, see #20189 (comment)