Without a screenshot is hard to say;
I remember to have seen this behavior only once before, when I forgot to put a muffler hatch. Maybe yours got obstructed?.
I'm using a Large Bronze one on 5.08.27, and it works fine...------
Speaking about the muffler, Blood, will pollution ever be a thing on the 5U?
Probably. If I can talk him into letting me do it, it will happen soon :p