I don't remember you telling us this happened with your New Imax...I remember reading your first post on WWDX. You gave us some information that showed your new antenna was working fine...but is showed to be higher in frequency...in the 10 meter band.
I have heard from others and seen better matches on my Imax. Maybe your antenna as mounted low or to close something.
I'm looking for a
full bandwidth curve that accounts for all frequencies <2.00:1 SWR on both sides of resonance. I have a notion the Old Imax shows most of its very wide bandwidth close to 5 MHz is below the CB band. I also suspect the Imax
is going down a long ways before it starts to rise again and maybe this has something to do with it being somewhat multi-band.
IMO 10 meters has always been the peak of the workable bandwidth on the high side for CB version of the Imax. My Imax manual has a Frequency/Antenna Length Table in the back for trimming up to 30.204 MHz and, IMO this supports the limit I suggest.
Does your manual still include the table?
To be clear, I see a slight trend like i've noted above in your analyzer image below...with the SWR rising toward the top of 10 meters. There also appears to be the development of a straight line curve at 1.50 SWR going down in frequency. Can you expand the range in order to see much more bandwidth until the curve starts to sharply rise up to <2.00:1 SWR on the low side of resonance?
I posted a very old hand drawn BW curve (PDF file below) for an old Imax that I plotted in 2009 using my old Autek VA1 analyzer. Back then I did the same thing as I see you doing below using your analyzer but I had to do it one data point at a time. I stopped my scan at 25.705 Mhz. I regret that because it was possible much more bandwidth near 1.40 SWR was never noticed. Sorry my image is in landscape mode and is sideways. Maybe you can use the PDF zoom feature for a closer look.
This chart shows best resonance for my Imax at 27.505 MHz...a bit above CB with a 1.1:1 SWR.