I had the same issue, but what ended up working was I (on my xbox series x) joined my PC player friend's party. After that, we could invite and see each other online. It's a weird requirement, but that's what worked for us. Before that, I could see him online, but he couldn't see me or join my party when I sent him invites.
Once the console players join your party once, the issue SHOULD disappear.
Found another solution turns out my buddies series x installed the regular xbox one version when someone else on a series x invited him it prompted him to install the series x version of the game and then me(on pc) was able to see him in game.
