Good question - and I think the test as described has a confound in it.

Two things change between a sharp 90° corner and a swept (radiused) inner conductor:
1) the local impedance discontinuity at the corner, and
2) the current path through the bend, so the conduction cross-section and current density differ.

The thermal camera only measures the result, not which mechanism caused it. A sharp corner crowds current at the inner edge of the turn, so localized I²R heating rises even if the impedance bump itself is small. A swept conductor spreads that path out, so it runs cooler for the same through-power.

If you want to separate the two effects, I would approach it this way:
- Run a TDR on both adapters with the same reference plane. That shows the magnitude and the length of the impedance discontinuity directly, rather than inferring it from temperature.
- Compare return loss across the band. If the swept part is genuinely better matched, you will see it above the frequency where the corner length becomes an appreciable fraction of a wavelength.
- Drive both at identical through-power and let them reach steady state. A single thermal snapshot is easy to misread if one part had better thermal contact to the housing.

So: not a wrong observation, but not a clean test either. Set up that way it mostly tells you the sharp corner runs hotter - which is expected - without proving whether that is mainly an impedance effect or mainly a current-crowding effect.