I have the same problem, my bet is that it is a licensing issue between the two parties.
I’ll take Destiny 2 for example. I started on Xbox and moved to PC later. And for those that don’t know, Destiny 2 has cross-progression enabled. The thing is that the DLC that I bought on Xbox wouldn’t carry over to Steam due to Xbox issuing the license for the DLC. I still had loot I had acquired from those DLC missions, I just didn’t have the missions. I ended up having to re-buy the DLC on Steam to play those missions on PC.