I'm experiencing this issue in the Create Above and Beyond Pack, Minecraft version 1.16.5, Forge-36.2.8, with version 1.3.5 of the Portality mod installed on a privately hosted server.

Setting icons with most blocks results in an empty icon when viewing that portal on the controller. It does display the message for changing the icon at the bottom of the screen, but the icon will appear blank afterwords. Sometimes, the icon is set and visible briefly but usually becomes invisible after some time.

For example, in the image below, I had just set the icon for "The End" to end stone and teleported to another portal, where it is showing as blank.