Southwire M550 says coaxial cable is misfired and reversed?
From what I can determine from the instruction manual for the tester, and my previous experience as a cable television technician, the test requires a free-floating cable in order to be tested properly. Your question regarding a splitter is valid, as there are electronic components within a splitter that will generate failed tests. "In the old days," one attached a 75 ohm terminating resistor to a piece of coax being tested and read the opposite end using an ordinary volt-ohm meter. If the reading was approximately 75 ohms, the correct cable was identified and was possibly healthy.

If the cable was already confirmed as correct and the reading appeared shorted, it was likely there was either damage or an in-line splitter, typically buried in an attic or nearly unreachable in a crawl space!

You can confirm the reading you see if you have a splitter in hand, along with a known-good segment of coax. Connect only the coax, test it (as good, of course), then attach the splitter and observe the difference.