You are not giving details about how you make changes so I will guess.
When importing a Datasmith file, Materials are created at a given path in the asset browser. Upon reimport, if Datasmith needs to create a Material at the same location (which happens 95% of the times), the old material will be overwritten. Therefore any changes made to material parameters will be lost on re-import.
If you are planning to make material changes in Unreal that you want to preserve when re-importing the model, duplicate (Ctrl+W) the source material and apply the newly created material to your objects.Those duplicates will not be affected when you re-import your data.
If you elect to duplicate and rename a material, consider giving it a name that acts as a reminder that it is ‘disconnected’ from the Datasmith Scene, or move it to a different path in the Content Browser.