From Batman News:
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As you know by now, Henry Cavill had a mustache for many of his Superman scenes in Justice League due to the extensive reshoots that he was part of. He had grown the mustache for Mission: Impossible – Fallout, which he was shooting at the same time. The mustache was covered up with CGI in Justice League, but many fans were still able to notice and felt that the final result was distracting.
Mission: Impossible – Fallout director Christopher McQuarrie was a guest on Empire’s podcast to talk about the movie, and he went into great detail about the whole Justice League situation.