Issue with SPFx part and OOTB Parts in WebView mode

I've discovered that recently, there appears to be a break introduced when viewing SharePoint modern pages with the parameter of env set to WebView(case-sensitive), e.g /SitesPages/Home.aspx?env=WebView

I have to different tenants, which presumably have similar versions of SPO associated with them, but in one environment, I cannot display a Custom SPFx part, whereas the other, it does display. The underlying code for the SPFx part is identical in each case.

When attempting to render the SPFx part in the environment where it doesn't work, the following error is visible in the developer tools:

Uncaught (in promise) Error: No callbackHandler found.
at e.sendMessage (sp-pages-assembly_en-us_763c3cb5e5e8ad1d42d26be71a7d7293.js:79:1269499)
at sp-pages-assembly_en-us_763c3cb5e5e8ad1d42d26be71a7d7293.js:79:1628439

In my SPFx part, I can see that the render function is called, but the promise for that render event is never fulfilled, causing the operation to die without any additional indication of a problem.

I've also tested OOTB parts, and determined that the Microsoft Forms part is also not working in the same environment where this is broken, but is functional in the environment where the custom SPFx part is working.

Please note, there are other tenants where appears to be a rolling change where the break shows up.