Here's the deal ... the "Southern Charm" star might've recently been involved in a DUI bust in South Carolina -- but there's another case that has ties to her (or her vehicle, more like) in the same state ... in which her car is alleged to have been involved in a hit-and-run.

This happened way back in October -- Kathryn was identified as a suspect in a hit-and-run investigation after a sheriff’s deputy was injured outside an elementary school ... although she was never arrested or charged with a crime because they couldn't pin her as the driver.

According to a police report, obtained by TMZ, Kathryn's car was determined to have been the vehicle in question -- but like we said, they never busted her ... and law enforcement sources tell us they're still not prepared to do that, even now.

We're told there hasn't been much movement in the investigation ... but the cops are still open to taking new leads and pursuing them if they arise -- so she ain't out of the woods.

Of course ... that's probably good news for Kathryn -- namely, that she seems to be in the clear there (for now) but she's got a whole new can of worms to deal with in a recent DUI.

We broke the story ... Dennis was arrested for an unrelated DUI last month following a three-car collision.

Shocking dashcam footage shows her emotional reaction to the arrest including threatening the arresting officer. In docs obtained by TMZ, cops say Dennis had "glossy eyes and an odor of alcohol coming from her person."