If you click on the red signature it should state specifically why it was showing red (i.e. expiration only). So electronically you can still verify the sig and the ink technically didn't disappear but is just qualified.
However, you have a good bug in the sense that when printing it shows fully invalid instead of something more granular like "Certificate valid, but expired." You should file this as a bug with MSFT.
Note the reason why this is possible at all is that in the electronic world this is a safeguard to ensure allow a bogus certificate to be revoked or expired, and other such corner but critical cases.
Shiraz Cupala / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge