The normal pattern on such an installation is to have some of the exhausts grouped to exhaust at the sides, and others underneath.

I don’t see why being at sea-level has any connection with running rich, nor why this should happen with any properly-handled engine. Obviously it does at times, for various reasons, but I don’t see this as being in any way normal. After all, Yak 11s have been around for a longtime, even in the West, without this being commented on. Do you ever see this on Sea Furies? Fw.190s? (OK, not so much evidence for those ones nowadays…)