<sh-select name="tz" [options]="timeZones" [(ngModel)]="project.tz" required
[placeholder]="'Timezone'"></sh-select>
I realized that the timezone setting did not validate when sent to my REST endpoint. The reason being that timezone was set as an array with one item. Should be set as the value of selected item.
For now I had to create a workaround in the project.service to send it as a string and not array:
project.tz = (project.tz.length === 1) ? project.tz[0] : project.tz;