Steps to reproduce:

1. Create an item, that should run ONLY hour 6 - and ONLY one time, next day, it should run again

Result:

Cant be done - The scheduled item doesn't run if Update Interval is set to 0, but If update interval is set to 1s, 1h or 1 day, the item are run in parallel, that is as described in manual.

I would suspect that the attached screenshot Single Item.png is actually the correct approach, but the calculated item, newer calculates.

Expected:
See screenshot....