@Ming Chan Thank you for reaching out to us, As I understand you want to remove the shading effect whenever you have background image via Company branding.
I did a quick repro regarding this scenario, noticed the same behavior, on further research, found shading effect would be present, not possible to remove it, in the past couple of customers reported this and we have raised a change request for the same but there is no ETA for this.
Would request you to share your feedback over here - https://feedback.azure.com/d365community/forum/22920db1-ad25-ec11-b6e6-000d3a4f0789 which is closely monitored by our product group team.
Let me know if you have any further questions, feel free to post back.
Please remember to "Accept Answer" if answer helped, so that others in the community facing similar issues can easily find the solution.