I’ve noticed that on microwaves that have the knob style timers rarely complete their cycles if you set it for anything less than 50-65 seconds. This behavior seems to occur irrespective of brand or age. Could it be that there is a single source for this component?
Those types of timers are spring driven. In order to set a small time, you need to turn the knob, typically, past the two-minute mark, then set the time you wish, otherwise the spring will not gain enough tension to drive the mechanism.
Or in other words, RTFM. 