I have confirmed that it is currently not possible to mirror your video while using the Slides as Virtual Background option. I'm very sorry about this inconvenience! I will be asking our documentation team to include this limitation in our article on the feature. I am however thinking this by design, as the mirrored video is only visible to you (meaning participants always receive the non-mirrored version). This could cause you to lean in the wrong direction while sharing and cover a portion of the presentation, as you would not be seeing the same video as the participants.
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