The Steamworks plugin currently has a workaround to make the Steam Overlay work. This is described in the Steamworks plugin documentation.
This workaround creates transparent window on top of the game, simply to be able to render the Steam overlay.
From what i've seen, it adds the D3D11Overlay_x64.ext.dll to your exports, which is a library used to create this DirectComposition Direct3D 11 overlay-frame which Steam hooks for the overlay.
While this makes the Steam overlay work on Webview2, it also breaks direct game recording with any recording tool, not just OBS. As they now record this transparent window, instead of the actual game.
For OBS, the recording can be fixed by changing the OBS settings.
There's also an open pull request to get OBS automatically to default to these settings for Construct 3 Webview2 exported games. See here:
github.com/obsproject/obs-studio/pull/13477
However this only solves it for OBS. For other recording tools you still would need to either record on a "Desktop"-level instead. Steam recordings definitely won't work.
All you can do on your side is deactivate the Steamworks plugin overlay setting, that would probably prevent this overlay workaround. Or not using the Steamworks plugin at all.
There's not much else you can do, other than reaching out to Steam support and beg to support overlay for web based games. Because Scirra is definitely not interested enough to get this resolved for us. As they've already spent too much time on the actual overlay workaround.