Basically, if you spawn a boss mob, either with or without AI, they cannot get effects applied. This has been tested in 1.7.2-1.7.10, with the same result (Only with AI; so it took a bit longer, but I can confirm it doesn't work either)

Obviously this bug being fixed won't make much difference in vanilla, but it's key for the many "BossBar" APIs in modded servers to work. I hope you consider fixing it soon