I know Shure claims that any channels within the same group will work together, but why does WWB say differently?
My guess would be that WWB is somewhat paranoid by nature.
My guess is the spacing settings (from other carriers and intermods) in WWB6's equipment profile for those ULX's are more conservative (or paranoid) than those initially considered to build the groups the units come configured with.
You can help WWB'S paranoia by tweaking the ULX equipment profile (in the Tools menu) to custom values that you find safe based on your experience with those receivers. Receivers with good selectivity reject carriers/IM close-by better than others, so spacing can be shorter, and you can then cram more frequencies in the same bandwidth.
I have no experience with the ULX's so I wouldn't know, but the "factory default" values for the ULX profile in "More Frequencies" mode is:
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channel spacing 425 kHz
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2T3O 250 kHz
Your screenshots show no signs of "channel to channel" incompatibility, so it's all IM spacing that produce conflicts.
Open one of those conflicts (press the ">" symbol) in the analysis to understand what frequencies produce the fault with that specific carrier, calculate the arising IM product (2f1-f2), and that IM separation to the your imcompatible frequency. It should be of a lower value that 250kHz...