Safari (iOS): unexpected caching behaviour

Hello,

We are the developers of Mail Service mail.ukr.net. Recently our users started reporting strange issue with using our Web service on Safari. Most of them use iOS 10.3.1.

The manifestation of the issue is following: version of source code of the index page (https://mail.ukr.net/touch) is older than the version of JavaScript resources included on that page. In the meantime we do not see any requests to our server for the index page according to user reports. But when attaching smartphone to Mac, in debugger we see a few month old version of the index page while source code, included via "script" tag, is topical.

Users clear all Safari caches but the issue persists. It seems that the issue is related to changes in the internet connection. Switching Wi-Fi hotspots or switching from Wi-Fi to 3G.

Note that almost all users had "Automatically Save Offline" option enabled in the Safari settings. Disabling it didn't help.

If not from our servers where else could Safari retrieve old page version from? Is there another cache that is not cleared from settings? Like on the iCloud, because of "Automatically Save Offline" option was enabled at the time of opening old page?

If so what extra HTTP headers should we specify to disable page caching? We currently specify:

* Pragma: no-cache

* Cache-Control: no-cache

* Expires: (time value of the past)

Best regards,

Team of mail.ukr.net