As of r502 we have implemented an experimental new mode to improve Steam support with Windows WebView2 exports. This thread has more information, and we're interested for your feedback on how it works out for you.
When you export to Windows WebView2 in r502, checking the Export for Steam option enables the new experimental mode, which we call "capture mode". This captures frames generated by WebView2 and copies them into the main application window. This means Steam sees the main app process rendering the actual game content like it expects. Therefore the Steam Overlay works, as well as other features that depend on using the window content, such as video recording from the Steam Overlay. This is an improvement over the old workaround which relies on rendering a transparent surface over the app for the Steam Overlay to render into - that works for showing the Steam Overlay, but it means other features like video recording capture the empty transparent surface, and so they don't work.
Note that to use this new mode, for the time being you must also uncheck 'Enable overlay' in the Steamworks plugin. The reason for this is enabling that still activates the old workaround using a transparent surface over the app, which will conflict with the new workaround. If the new experimental mode turns out to work reliably, we will publish an update to the Steamworks plugin that removes this option so the old workaround is no longer used.
Note that the new capture mode logs its status to the browser console, as well as any error messages that were encountered. If anything seems wrong, or you just want to check what its status is, check the browser console as usual (make sure dev tools are enabled on export, and press F12 in your app to open it).
We're keen to make sure Construct's Steam export works as well as possible despite the limitations of Steam's support for browser technology and Valve's unresponsiveness to requests to improve it, and we've gone to great lengths implementing this new mode to try to make sure it can work better. So please give the new mode a try and let us know how it works out for you in this thread.