Tanya Sam -- who's not an official Housewife but a "friend" of the show -- has officially bailed on filming the rest of the current season ... production sources tell TMZ.

As for why ... well, we're told it revolves around Cynthia Bailey's bachelorette party and what Tanya and Porsha Williams did with a male stripper while cameras were rolling ... and what they may or may not have done when cameras shut down.

Our sources say Tanya's not happy about the alleged stripper sex incident being so heavily promoted for the upcoming season, so she stopped filming about halfway through.

We're told she had other issues too -- like her relationship with her fiance and his alleged cheating taking center stage on season 12 -- but the stripper scandal was the last straw.

Our sources say she hasn't officially quit 'RHOA,' but she's done for the new season ... so viewers will notice her drop out after a few episodes.

We're also told that as of right now, Tanya's so ticked off she's not even considering doing the reunion show when the season wraps up.