Tecsun PL-880 SSB Fine Tuning problem

Some Tecsun PL-880 made in 2015 present a tuning bug when using the Fine Tuning knob in SSB . This short video shows this bug. Here, my HP RF generator is on 14,100.5 MHz at –107 dBm. The PL-880 is on 14,101 MHz, LSB mode. The AF beat frequency from the PL-880 is at 500 Hz At the beginning, I use the Tuning knob to tune up the PL-880 to 14,102,00 MHz. Of course, the beat frequency increases and is now at 1500 Hz, and this is normal. Then, I use the Fine Tuning knob to tune up, but now, the beat frequency