The left thrust lever has operated correctly; the problem is that in response to a thrust reduction command, when the climb mode of FLCH/VNAV - THR; changing to ALT CAP -SPD, that gives a thrust reduction.
The aircraft was initially cleared to climb unrestricted to FL290. At 14:39:01, ATC instructed the aircraft to stop the climb at 11,000 ft due to traffic. The preliminary report states the L thrust lever started reducing at 14:38:42, ie BEFORE the aircraft was instructed to stop the climb. Does that not rule out a thrust reduction caused by an ALT CAP-SPD mode change?