I see this as well (build 1.21.12). The math being done doesn't seem to be super consistent- using the Trade Port, Heavy Factory & Autocannon Defense Platform I see the %-reduction span across [8%, 9.33%] for the cost reductions. It is listed as a 10% drop though (poking around the research entity file).
Edit: checked this on TEC Loyalist homeworld at game start; so no planet bonus shennanigans are occurring.