There is no hard and fast rule. Indeed many WG chairs have work or personal
interest in the area of the group (how could it be otherwise?) and occasionally
write drafts.
However, the preference is not to be an author. In particular, if you are an
author you should not be the chair deciding or document shepherding about the
topic. This is one reason why we have multiple chairs.
In general, it is easy to get into a situation where it is difficult to
progress a document because all the parties who should do something about the
document are recused due to being authors. Having other people hold the pen and
the chairs doing their chair job is a far better model.
Jari