Loopback not overdubbing on audio tracks

Loopback is behaving erratically on an audio track I have with a microphone input. I want to record 1 bar of vocal backing, and then immediately overdub a harmony on top on the second pass without stopping recording, but as soon as it returns to the start of the loop, it wipes what I just recorded. If I leave it recording, and attempt to record other vocal parts for the sake of seeing what it will and won't do, it suddenly decides to remember that pass, then will wipe the next one, or the one after that, with no warning of what it will keep and what it will clear.

It is maddening. It seems the basic and traditionally expected function of a looping pedal, namely that of being able to leave the thing recording while you overdub on subsequent passes, is completely unreliable.

Please, please, please could someone fix this? I can't imagine it would be that tricky since Loopback instances on instrument channels overdub perfectly with no trouble.

Sincerely

Alex Campion