Just to close this out, FlyWithLua has a built in module called radio.lua which executes the following code, amongst other things for consistency within scripts:
dataref("COM1_STDBY", "sim/cockpit2/radios/actuators/com1_standby_frequency_hz", "writable")
dataref("COM2_STDBY", "sim/cockpit2/radios/actuators/com2_standby_frequency_hz", "writable")
Setting those to "readable" will solve the issue, but doesn't help for those that intend to write to the datarefs from their Lua scripts. Using the 8.3 khz spacing datarefs will also fix the problem.
dataref("COM1_STDBY", "sim/cockpit2/radios/actuators/com1_standby_frequency_hz_833", "writable")
dataref("COM2_STDBY", "sim/cockpit2/radios/actuators/com2_standby_frequency_hz_833", "writable")