This is definitely happening. A engineer can't really work on anything truly complicated if they hop around every 2-3 years. These types of engineers gain cursory knowledge of topics but never become experts.
But about your point I think it can depend greatly. Personally I switched jobs many times but have so far kept in the same field and my knowledge has compounded and I have never dealt behind. In fact sometimes I feel the opposite, some coworkers who have less diverse experience have less to offer.