Hi.
It happens beacuse in your form you have to fields with storage standart / article / alias.
If you want to constract alias from to fields, but have control over it you have to do next:
Create field "alias text" with storage standart / article / aliastext
Put id field in this form.
Create SD Conctat field with storage standart / article / alias and put "alias text" and id fields in SD concat. Process on edit - YES.
For SD concat field set variation to "value".
In this way you will have more control over alias because there is separete fields for title and alias and