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If there's a legit beef between Millie Bobby Brown and her "Stranger Things" costar David Harbour ... they clearly decided to play nice in front of the cameras at the premiere of the show's final season in L.A. last night.

Here's the deal ... according to recent reports, MBB allegedly filed a bullying and harassment complaint against David, apparently during their work on the final season. Neither of the actors -- nor Netflix -- addressed the situation prior to Thursday night's premiere.

Millie Bobby wasn't asked about the alleged feud ... but did talk about her "special bond" and journey with David all these years. Gotta note, the two were all smiles when they posed together for pictures.

Shawn Levy , executive producer and director of the 'ST,' was asked about the feud

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