You can rule out the App, for obvious reasons...first being it is reporting that your radio signal is the problem, second is it has nothing to do with flight distance anyway. Shooting problems like this are just to much to find the error, and you have to start removing things from the equation. If it was indeed a firmware problem then everyone would be reporting the issue, or a LARGE number of people. Adjust antenna warning could mean a few things, but most normally your not pointing them at aircraft or close approximately in general direction.
Given that you are flying in a once known "good" location..you are going to have to deduce that it is something now in that location that has changed...or that your having a hardware issue (Defective).
Rule out location, go to a different (hopefully radio LOS) and climb to say 300AGL shoot it out in distance and don't vary direction. See if it reports same issue. If it goes substantially farther than previous than most likely your system is good.
So now you have to figure out if some other source is influencing your known good area now...what that is could be hard to determine as it might not be there all the time or may be atmospheric in nature. Your just going to have to use some basic troubleshooting as I outlined and narrow it down I guess. I fly a primary path here to go out scouting....different days have different outcomes, as long as you rule out hardware failure, that is the first step