What car and where have you placed the external mic? Echo (too sensitive ) and low volume (not sensitive enough) are contrary so I'd suspect a mounting issue. For instance if vibration is induced in the mic, the noise cancellation will try and dampen it and that could have side effects.
I don't think it's a bug in the system, on the other hand I'd also appreciate a gain control as echo occurs in other installations using the DUDU7 supplied external mic as well and that could be handled by slightly lowering the gain.
I had to do some adjustments (had echo) but use the OEM mic of my VW, quality is perfect now (lowered supply voltage of OEM mic - this fix is irrelevant for DUDU supplied mic though).