TOKYO—Japanese artists hired to promote Walt Disney Co. ’s movie “Frozen 2” on Twitter said they were instructed to hide that they were paid for the work, contradicting Disney’s initial explanation that the omission was accidental.

“The agency that hired us for this campaign requested that we not label our work as a promotion,” said an artist who publishes under the name Kosame Daizu in a tweet.

The artist, who has more than 100,000 followers on Twitter, tweeted a cartoon last week in which he depicted some characters in “Frozen 2” and reviewed the movie favorably. He didn’t respond to requests for comment.

Other artists involved in the campaign also apologized. An artist who goes by the name Yamato Nadeshiko tweeted that an earlier tweet praising “Frozen 2” wasn’t labeled as promotional in accordance with the client’s directions.

Disney initially said it had intended to ask the artists to show that the tweets were part of an official advertising campaign, and it called the absence of disclosure a lapse.