I've seen many examples of non-root CAs for such purposes, but it seems unusual (though not completely unheard-of) among root CAs. I dug through Mozilla's standard certificate bundle, and found very few such certificates. Amazon has one, but they also use that to issue certificates through AWS. Someone elsewhere in the thread mentioned a DoD root CA. The certificate store has some certificates from companies like Dell; no idea what they use those for. Who else has a root CA used exclusively for first-party certification?

In any case, I was only trying to suggest why this seemed odd, based in part on the availability heuristic: whether this is a common practice or not, it doesn't seem like something people (even people who regularly read about changes or events in the CA landscape) would see regularly.