Do you tip MUA?

I didn't because she owned her own make-up/cosmetics company.

I'm going to a studio with multiple MUAs.

My rule of thumb for tipping is typically if they run their own business I don't unless they do an exceptional job. If they work for someone else than I would tip them. In this case yes you should and it is expected that you tip. Between 15-20%.

So if the service is $400 then I'm expected to tip $60-80?

Yes. Keyword expected. Its not mandatory.

Mine was my regular stylist and did my makeup for free. I still tipped her because she is amazing.

I always tip makeup/hair, own business or not.

Mine is a company owner as well, but her contract distinctly states that gratuity is still expected. At least there are no surprises for me, I guess.