Studies are fine and a normal part of artists development, but as Jaguar said, it needs to be stated that it's a study. Also in OCA studies of other artists work isn't allowed for anyone who sees this information.
On Hive we have to be a bit extra vigilant about enforcing rules on plagiarism and things like that because the incentive for people to do that is high. The community has a long history of people ripping off a ton of money by claiming others work.
I think what @artizm did was an honest mistake and welcome her to continue to share her work and just be mindful of the rules in the future.