Interestingly enough I’ve been searching for this issue for the past few weeks. After some testing I finally figured out the problem and the simple fix. It seems that after the driver is installed, Windows (in my case Windows 10) will default the playback of the Behringer USB to 48000Hz. (Go to Playback Device > Behringer device (such as UMC202HD) > Properties > Advanced). In my case it defaulted to 16 bit, 48000Hz - CD Quality.
However, Audacity defaults to projects working at 44100Hz.
So with a mismatch I had the issue you and others have described where you can record the first track but then trying to do overdub with playing that first track and then recording a second track, nothing happens.
I’ve set the Playback of the Behringer to either 16bit or 24bit 44100Hz to match what Audacity uses and now overdub (recording while listening to previously recorded tracks) works perfectly.
Yeah!