MQTT single-level wildcards ("+" character) are broken within topic subscriptions. This is due to URL encoding/decoding that is done on the component URI, which results in the "+" being replaced with a space. I also tried with "%2B" instead of the "+" sign, which gave the same result.
A search led me to
CAMEL-8056, which mentions needing to override the useRawUri method to return true. I'm working on a patch and updated unit tests for this.