CO01 - system changes sch.start time given by the user (fwd sched., no cap. check, no shifts defined)

Question

Dear Experts,
I am facing a very strange issue. Whenever I am trying to manually create and schedule a discrete production order  in ECC 6.0, with 1-forward scheduling (in CO01 transaction), by giving scheduled start date and time and pressing the "Schedule" button; the system changes the start time given by me (the date remains) and executes the scheduling with that start time.
For example, when I want to produce 12072 PCs, and I give 08:00 as shceduled start time, the system changes it to 15:59, adds the operation times correctly and defines the 'correct' scheduled end date (again, with fwd scheduling type!).
There is some logic on how the system changes the scheduled start time given by me. Below a certain production quantity, the issue doesn't come up, lead time scheduling is carried out, as it is expected. So the problem only occurs, when the order qty exceeds a certain limitation. In that case the order cannot be scheduled to a certain time phrame (between 07:59 and 15:59), so the system changes the scheduled start time given by me (whenever the order falls into the mentioned period). Depending on the order qty the mentioned period (to which the order cannot be scheduled) is changing.
I made the following checks in the system:
production calendar is set correctly, it is not blocking the production
all the applied work centers are available from 00-24 hrs
no shifts has been defined on any of the WCs, production is enabled in 00-24 hrs
there is no other production scheduled on any of the work centers, capacity load is 0% on all of them, as it has been checked in CM03
order scheduling was performed with '1-forward' scheduling type
anyway, capacity check is switched off in the system
formulas on the work centers are all correct, the processing time calculated based on them is also correct (No queue, wait, setup, etc. times are set in the system, we are only using processing times.)
no error messages
As far as I know, in the above case lead time scheduling must be carried out, the system is simply 'not allowed' to change the start date given by me, just add operation times to the start and define the scheduled end date.
Thank you in advance,
T

Answer 1

Jiaul,
please find the screenshots.
1.Mark for Relevant finite scheduling in scheduling or capacity tab - yes
2.Can be used for multiple operation - yes
3.Capacity Planner Group set up - yes
Caetano,
in my case the order scheduled start date is exactly the same as the operation start date. The problem is that this date cannot be manually scheduled to a certain time interval, the system changes the scheduled start date specified by me. (Like there was a "blocked period".)
Thank you,
T