It seems users trying out SOULCALIBUR VI in Steam Play ended up with a major issues in both Linux and Windows due to a bug in Steam Play's Proton. This bug, ended up making it so SOULCALIBUR VI would not be able to use any online features after loading it up on Linux with Valve's Steam Play even once.
I held off on reporting about this previously, because many things about the problems were unclear and I didn't want to end up making anything worse. This has, thankfully, been cleared up now by Valve. Here's what Valve developer Pierre-Loup Griffais said on the bug report:
The issue with the SOULCALIBUR VI network error is now understood to be caused by a bug in the Proton Steamworks implementation of leaderboards data. The bug causes the Steam client to upload corrupt entries to the backend, which causes the game client to fail network initialization on all platforms. We will be deploying a fix in Proton as well as deleting the corrupt data for affected accounts. Due to the design of online data for that title, it also appears that affected accounts end up fixing themselves after a couple days of not attempting to log in through Steam Play. It's possible there are more network issues masked by this one, but none are known as of yet. There are also other issues affecting this title at the moment, such as missing FMVs and occasional client freezes on startup.
As with any issue specific to Steam Play, please remember to get support directly from Valve through this tracker, rather than reaching out to the original developers directly. This is especially important in this early beta stage, as these issues are largely outside their control, and developers of Windows titles are not typically equipped to deal with these issue reports.
Of course, this is a good reminder that Steam Play is still relatively new (even if Wine itself has been around a long time) and so there's probably a number of major issues like this waiting to be found and fixed. Still, it's good to know Valve are on top of things before this ended up becoming something even bigger for other titles.
It's also a good reminder to report issues to Valve not the game developer, as Valve did ask in their original Steam Play announcement which you can do so on this GitHub.
In other Steam Play related news, for those wanting to try The Vanishing of Ethan Carter Redux on AMD GPUs it looks like the next version of DXVK will spoof an NVIDIA GPU to fix it. It also seems that Quake Champions may also get a performance boost, Sword Art Online: Fatal Bullet is going to spoof an AMD GPU to improve performance and a few more little bits like that should come.