I just tested on Android tablet (Nexus 9) and an Android phone – and it’s the same issue.
1) The Android tablet(!) seems to load in BOTH the tablet.css AND mobile.css, so the issue is not iPad specific.
2) While the Android phone is ok, it only loads mobile.css (iPhone is ok, too)
maybe the correct word is not subpage, but parent-child page relationship.
That’s what I meant – that if polylang is on, then the child (subpage) becomes a main page.
I did check all the settings, and all synchronisation is off.
Thanks for any help!