A few others have noticed this. It seems from their posts that this only occurs
when using digital modes that interface the uBITX via analog audio input.
SSB mode uses a different signal path than plain CW on a uBITX. If you
are not seeing the RF output level variation using CW but are seeing it during
digital modes with audio interfaces, it may be that the audio input is changing.
If this is the case, then it may be that the input is changing frequency and driving
your RF off frequency far enough for the IF filter to be blocking part of the
signal, or that it is driving far enough off frequency for the RF PA filter to be
affecting power output. Both level and frequency are critical for digital modes
that are introduced to the uBITX as an audio signal.