Occasional loss of a requiredFile in HTMLView

For some months I've had occasional examples of this behaviour: the Dashboard is open and an onSave trigger on the daily note correctly asks for an update to this display.

The right-hand part here is what is displayed. All the text is correct, and most but not all of the formatting is right.

As you may remember after generating the HTML and sending it to the API, I then persist a copy of it to a local file. The left-hand part here is what happens when I render that in Safari (my usual way of debugging complex HTML). This shows it completely as expected.

This happens I reckon about 1 refresh in 60, but I can't find more of a pattern than that. When I refresh the Dashboard it always corrects itself.
There are no errors or warnings in the log. Specifically this is happening on b1085 on my main machine; I'm keeping that on the 3.9.5 release to avoid the other major bug.

I've reverse engineered what is happening. There are two sources of CSS:

1. contained in the main HTML that's generated.
2. comes from a static dashboard.css file which is one of the "plugin.requiredFiles" defined for the plugin.

If I manually disable all uses of that file in the Safari debugger/inspector, it then looks exactly like the wrong image above. Therefore, I'm 99% sure that occasionally when an existing HTML window is being re-used, a requiredFile is somehow not read or applied.

Originally discussed in Discord thread