Hello Leandro
What you observe on your system is according to the standard system design.
When you run transaction MD40, system check the planning file and only plans materials for which the planning file flags are set. When a material is planned, these flags are removed and they are only set again when there is an MRP relevant change for this material. You can see the planning file on transaction MD21.
When you plan a material on MD41, however, you will manually input the material number. Therefore, this material and it's components will be planned, even if the planning file flags are not set. This is standard and it cannot be changed.
See the information below provided on help:
In single-item planning, you can only choose between net change planning and net change planning in the planning horizon. Regenerative planning is not useful, because the material has already been defined and does not have to be determined after the evaluation of the planning file.
BR
Caetano