This is peculiar. The envelope icon appears to generated with blue lines and "transparent" instead of white for the fill. (Favicons support transparency.) The area outside the envelope should be transparent, but the area inside the envelope should be white. I would call this a "bug." Is that supposed to be reported here or somewhere else?
Here are screen shots when Groups.io is the active tab and when its tab is inactive. Now that you've advised me to look closely, I can barely make out the little envelope. I'm not sure you will be able to see it I had never noticed its appearance.
Larry
Laurence Marks