Unbound Avatar is not listed in support gems Supportable Skills list

I went to go rework some support gems and I realized Unbound Avatar (UA) is not listed in the Supportable Skills list when hovering over any support gem. I can confirm the support Prolonged Duration works on UA as it's socketable in the skill. It's not a huge problem, but for more casual players like myself it makes it a little tougher to figure out what gems work with this ability if it's not listed as supportable or not. See the screenshot for an example looking at Prolonged Duration II which I currently have socketed into my UA skill gem.