Mingming became a Pledis trainee sometime between February 2012 and June 2012, training for about 2 years before his departure in 2014.
On July 11th, 2014, Pledis Entertainment updated the Seventeen website to show new profile pictures of the pre-members and a new group photo. Along with the new photos, there was a line-up change. Mingming, along with other past pre-member Doyoon, had been taken off of the list of pre-members.
At the time, Mingming's departure has been speculated by fans due to his sudden frequent posting on his Weibo account, his creature of an Instagram account, somthing most Entertainment Agencies don't allow trainees to have, the fact that his picture updates showed him to have been in China for an extended period of time, and that he had an update in which he stated that it had "Been some time since [he] last danced".
(Being in China for a long time would be odd for a trainee as most trainees typically visit their homes for a few days at the most. And not having danced in a while would also be odd for a trainee as trainees typically practice long and hard to improve skills such as dancing.) While neither Pledis nor Doyoon have commented on Mingming's departure, a friend of Mingming's posted on Instagram saying that Mingming would "have a brighter future after leaving the company" suggesting that not only had Mingming left Pledis Entertainment (rather than just Seventeen), but that Mingming had also himself walked for the company (though the second is unconfirmed).
The specific timing for his departure is unknown, though the time he left is thought to be somewhere between March 14th, 2014 and May 21, 2014. This is due to the former date being his last appearance with Pledis, and current pre-member Joshua posting a selfie of himself at the profile shoot where the photos the would eventually show Mingming's departure were taken on the latter. The reason for Mingming's departure is also unknown, though some fans assume it was due to Mingming receiving unfair treatment.
(Seventeen regularly posted updates called "Seventeen Diaries". In a year of Seventeen posting them, Mingming had only written one diary, despite being one of the original pre-members. Also, for the Like Seventeen Show 3, the only stages Mingming participated in were group performances. There were three of these, out of eighteen total stages, one being the encore.)
Info from pledis 17 wiki
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