Previously, I was told by support and a developer on Steam forum that new DLC purchases will have both FGC and FGU as their parent if you own them both on Steam.
Today, I made DLC purchases and they only showed up in FGC's Steam DLC library. Of course, I can use the DLC on FGU thanks to Steam link, but seems like the Steam DLC libraries do not work as intended re: having both FG versions as DLC parents.