Hmmm...I just checked at this end and yes you are correct that the decimal values cannot be changed for some reason. I even checked the XML code for the ADF radio inside the .cab file and the code just has lines to change settings for the whole digits and not the decimals.
I am not sure if Roman coded all the radios but I suspect he 'borrowed' the basics for them from a Cal Classics aircraft (with full permission of course) as I seem to remember a discussion about the radios at the time. As there is no code to even control the decimal wheel I don't know how it displayed anything other than a zero.
While typing this I had a thought (dangerous at my age I know!
). Is it possible that this is a left-over from a previous aircraft you were in where the ADF radio had the ability to change the decimal digits and then the Boxcar picked up the last settings. I know Roman coded a persistent 'memory' function into the Boxcar where it remembers the settings of multiple 'options' (see p.39 of the manual) so that the aircraft is in the same state it was in when you left it but I don't know if that includes the radios or if the sim does that between different aircraft. Could you possibly run a test for me to confirm my theory? If you have an aircraft that does have the ability to change the decimal digits could you set it to a frequency 'XYZ.0' where 'XYZ' is a known and valid frequency and then switch to the Boxcar and check if the Boxcar ADF radio also now shows a frequency of 'XYZ.0'. If it does then I can only assume that the Boxcar radio is picking up the values of the last used settings for the ADF radio.
It won't fix the problem of not being able to tune the decimal digits but to be honest I can't ever remember tuning an ADF frequency in either sim (FSX or P3D) that ended in anything but a zero!
At the end of my flight the compass showed 210 degrees but the GPS was 180.