@hjuutilainen
Thank you for reporting this.
I installed Obsidian as user A and am using it as user A and B on my laptop. The icon was showing correctly when logged in as user A, but it was showing as a rounded grey icon (liked reported there) when logged in as user B.
I changed the permissions of Obsidian.app/Contents/Resources/Obsidian.icns to 0644, cleared the Mac OS icon cache for the user B using this and the icon is now showing correctly for both users 
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