Dustin,
In App Broker 2018R1 and later, there was a change made to the web.config for the ESD site to improve security.. Specifically, "nosniff" was added as a custom header to the web.config for App Portal. If you were to remove this custom header then the icon would display.. The security issue is described nicely at the following URL:
https://helmetjs.github.io/docs/dont-sniff-mimetype/
You could remove this new custom header from the web.config, but it is going to make the site slightly more insecure.. The typical work around would be to save the offending icon as a different type, such as .bmp, and then name it back to .png.. This typically removes any formatting which is triggering the behavior.. Note if you were to run Chrome/IE in developer mode, and viewed the console, you would see that an error was occurring when trying to load the icon.
It is important to note that this issue is related to the browser/icon, and is not directly related to App Portal.. The same behavior could be reproduced if you were to copy the icon to any site in IIS, if the nosniff header was set.
Charles
