[BUG] Fallback behavior differences from Chromium implementation

[jamesnw]

Describe the bug
In Chromium browsers, when a fallback is applied, it stays until the fallback overflows and a new fallback (or the initial position) is selected.

In the polyfill, when a fallback is applied, it reverts to the initial position as soon as the initial position no longer overflows, even if the fallback does not overflow.

To Reproduce
Compare the fallback examples between Chrome (no polyfill) and Firefox (polyfilled).

Expected behavior
I'm uncertain of the expected behavior and haven't been able to track it down yet. My reading of the spec aligns with the polyfill's implementation, but it may have something to do with the nuances of the timing that are mentioned.

I do not see any relevant spec changes.
I am not sure if this is a change in Chromium's behavior, or if this has always been the case.