Ok, my thought on this is looking at room size and placement. Specifically, your full wavelength there in that suckout is about 25’, and half- is 12.5’. Are there corresponding measurements between that sub and a wall? What is a little stranger is that the other sub shows it too, and also at an octave up, though the right doesn’t show the octave up.
Anyway, my solution would be to try breaking any symmetries up. If your speakers and subs are in line, move the right on into the room by a couple inches, and lefty back a little, or forward but not as much... try to get them so that they aren’t symmetrical to each other or your towers and recheck. If it is a frequency cancellation due to wavelength, you should be able to break that up.
I had to do that with my towers, and I got 10dB of a 20dB dip back. Just by moving my left side by about 1.5 inches on a ~22’ wavelength cancellation (11’ from driver to wall and back again... showed there, and at the octave!).