strange behavior with duration in minutes

I have set the duration in minutes with a step of 30 minutes:

gantt.config.duration_unit = “minute”;
gantt.config.duration_step = 30;

Some of my tasks have a duration of 1 and they are displayed with the correct duration in the gantt. But anothers tasks, with the same duration, extend to last several hours.

The start and end date are consistents with the duration.

Solved. It was because gantt.config.work_time was set to true