Many of those with the issue say they're with T-Mobile US and have speculated that it might have to do with the carrier's recent RCS messaging update. However, T-Mobile describes it as a Samsung problem. Samsung, meanwhile, told Gizmodo it was familiar with the reports and had its teams "looking into it." We've asked the company for additional comment. It's not limited to any one device, though. Numerous handsets are experiencing the random message flaw, including current-generation models like the Galaxy S9.
It's not certain when there will be a fix. In the meantime, though, your best choice may be to disable Messages and use another app, such as Google's Android Messages. Suffice it to say this isn't a minor bug -- it's at best confusing, and could require some very awkward explanations to friends and loved ones.