Why did Phlox's mouth start to exhibit this behavior?

I’ve been watching Star Trek: Enterprise and I noticed that, starting with Season 2, the left side of Dr. Phlox’s mouth no longer moved correctly. I looked up the actor, John Billingsley, to see if he had a stroke or something but didn’t find anything.

I also wondered if he had nerve damage similar to Sylvester Stallone or Milo Ventaglia, but his mouth seemed normal during the first season. In an interview filmed several years after Enterprise, his mouth again seemed normal.

Why did Phlox’s mouth exhibit the behavior shown below, and was it due to a stroke or any kind of medical condition on the actor’s part?