No, I haven't heard of it, but that doesn't mean anything because the only converters I've actually seen are the one I own and the ones in trailers in showrooms.
If I saw 27 VAC at the converter outlet, I would back away carefully and kill the supply breaker. I have to guess that it's possible something is shorted in the converter, and that can't be good. Another possibility is that this particular design is intended to be operated only with a battery
connected, and the battery
is required for output power filtering (without the battery
, the output would be a very spiky DC which could look like a higher voltage AC to some meters).
The Parallax web site
may have some useful information - it has all sorts of guides and diagrams, but you need to know your model number.