Back in August 2018, Valve announced their new Steam Play feature with the Proton software in the Linux Steam client to play Windows-only Steam games on Linux. A little note about what platform is counted for sales.
When we spoke to Valve originally back then (shown in an update to our original article), we asked about how the sales would show up for developers and this was the response:
Hey Liam, the normal algorithm is in effect, so if at the end of the two weeks you have more playtime on Linux, it'll be a Linux sale. Proton counts as Linux.
It seems that there might be some issues where it's not correctly counted, so it shows up as a normal Windows sale as a user noted on Reddit. Since reaching out to Valve, developer Pierre-Loup Griffais has released this quick and simple statement on Twitter for all to read:
That doesn't seem like intended behavior, we'll look into it. At this early stage, the team's focus is still on compatibility and performance, so it might take a little bit.
As with anything new and in constant development there's going to be teething issues. Hopefully this hasn't been too widespread though if true.
Update 19/02/20: I've now had this verified by a developer whose game I purchased on Linux, then played entirely on Linux and I know a few others who did the same and the developer told me all show up as Windows sales.