Thank you for sharing this!
Based on my research, this seems to be an expected behavior, unfortunately. It is definitely possible that Esri Support Services can log an enhancement request for you. At the same time, what worked for me was when I resized the image to 400x400 and saved the group logo (image), the background automatically became white (rather than transparent). This was with Windows Paint. In that way, when you use the newly resized image with white background, it is no longer gray and achieves the end goal.
After choosing to have white background:
I understand that this is not ideal but we can have a white/transparent-looking background through this.
Saka D
ArcGIS Analyst | Esri