Cache is saving Mixed Content assets.

Hello,

Unfortunately I cannot offer a public-facing link to the website in question as the site is live and I cannot leave it in a broken state. I can however describe the situation.

Swift is caching mixed content urls for my page assets (css and js).
Without swift, my site loads securely.
With swift, I get a bundle of mixed content errors on my font page.

(Mixed Content: The page at 'website' was loaded over HTTPS, but requested an insecure stylesheet 'https://website/wp-content/plugins/some-plugin/some-asset.css'. This request has been blocked; the content must be served over HTTPS.)

Interestingly, this occurs on the front page, but only after I have visited (swift has cached) any of the other pages on the website.

If I deactivate and reactivate swift, the problem goes away until I start loading other pages again.

Any idea at all why swift would be saving erroneous http urls?

Many thanks,
Brilliant plugin otherwise, certainly considering a license.