RawTherapee does not embed color profiles nor metadata in PNG files. It's not a bug. I don't know specifically about color profile support in PNG files, but metadata support was poor, meaning that even though the format allowed it few programs would read it. That was the case when support for PNG was added to RT. It seems to me that nowadays metadata support in PNG files in general has improved, meaning more FLOSS software is capable of reading and writing it than was the case a few years ago.