mattiL: Actually what you are seeing is sort of the "reverse" of the original issue reported by the OP. The original issue is that when a port, e.g., eth4, is put into "switch mode", the counters for that particular port is no longer reflected in the Web UI, which was actually a bug as the counters for "switched ports" are indeed available on the ER-X platform, and that should be fixed in 1.8.5.
What you are seeing is actually expected, since the "switch0" represents an interface for the router itself and hence its counters should reflect packets that come "into" the router but not those that are switched (at the switch chip level and do not go "up" to the router CPU).