I have been using the Microstrip Line and Coplanar wave guide with ground plane calculators to get 50 Ohm tracks.
The latter should converge to the same impedance Microstrip as track to top plane spacing “S” increases, but does not, staying about 4 Ohms higher. I have noticed the same “feature” using the online https://chemandy.com/calculators/coplanar-waveguide-with-ground-calculator.htm
The coplanar waveguide formulas are derived under an assumption that the gap width is not very large. You cannot assume they will converge to the correct value when taking S -> inf.
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I think that you are probably correct, the Chemandy page has a warning about H>>b to avoid modes.
There should be a warning on the KiCad coplanar calculator,
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