It looks like the ALT:RADAR call will not return the radar altitude if to high above a body. In my testing when descending to the mun ALT:RADAR would return more or less the same value as SHIP:ALTITUDE until within about 10km altitude or so of the surface, this will likely very as I think the change to actually reporting the radar altitude is because of the terrain fully loading in. I also observed ALT:RADAR not reporting the radar altitude once the craft went above around 30km altitude though this required descending low enough it actually starting reporting the correct radar altitude.

I suspect this is a result of us trusting the KSP API for this data as in my testing calls such as SHIP:ALTITUDE - SHIP:GEOPOSITION:TERRAINHEIGHT and the :BOTTOMALTRADAR suffix never stopped reporting what looked like the true radar altitude.

First noted by user gmaster0o0 on the kOS discord.