RC-500 Loop Station : MIDI In is Useless

I truly do not understand the point of this looper having a MIDI In port. It's meant to keep up with external gear and synchronise with the 'clock in' signal from other sources. In my case it's the MPC Live MK2, which works fine with any other piece of gear I own. The Boss RC 500 does not play along as a slave device, mangles the loop I've just made, making it sound unusable. Boss's advice is to disable Tempo Sync feature, which defeats the purpose of having a MIDI In, Clock Sync port. The unit cannot adjust to slight clock variations and therefore time-stretches the recorded loop, making it sound like it's skipping frames, all jumbled up. Also, if Tempo Sync is disabled, the recorded loop plays at random intervals, when the unit feels like it. I can't make any sense of it.

The unit does work as a Master Clock, however, so not all is lost.

I've tried using different devices as a master clock to sync with RC 500 as a slave but it doesn't work smoothly.

I think what Boss needs to do in a software update is to make it ignore the slight BPM changes of an incoming clock, or at least average across time, and disable the time-stretching algorithm, or at least make it an option to disable time-stretching without disabling the Tempo Sync.