That makes it sound like the "bug" (lack of support for updating TLS session keys) was known for a long time, and the original "fix" was the client admitting it

That makes it sound like the "bug" (lack of support for updating TLS session keys) was known for a long time, and the original "fix" was the client admitting it didn't really fully do TLS 1.2, which based on your experience seems to be a typical "fix" in SChannel. (or, I'll admit, it's more likely the fallback after a "failed negotiation" was a "robustness" behavior that essentially hid problems)