Numbers & Pages for iOS behaving strangely

Since approx week or so ago I have noticed the "Set Password" option for documents has completely disappeared from the settings option list. This is both on my iPad 4 (iOS 10.3.3) and iPhone SE (iOS10.3.3). Another peculiar behaviour is that if I try to reopen a document (Pages or Numbers) too soon after I have closed it I get an error message (screenshot below) advising I cannot open it as "owner stopped sharing...". Completely illogical, given that the device I use is signed into iCloud with same ID.

By trial and error have found that if I leave it and try again a few minutes later (OR if I force the Numbers/Pages app to close) I can reopen it without the error.

Existing documents that were password protected BEFORE this odd behaviour started still request password BUT I cannot add password protection to an existing non-passworded document as the option has literally vanished - see the after and before shots below:

I suspect Apple have been messing around with the iCloud/web versions of these applications as a follow-on from recent iOS 11 update but am surprised that an update like that would have such an effect on basic functionality on my iOS devices. Whether updating to iOS11 will resolve this I do not know yet but would prefer to know beforehand. If that is the case the BIG problem I have is that my iPad will not be updatable to iOS11. This, horribly, is starting to sound like Apple have decided, without prior warning, that it is time for me to spend several hundred pounds on new hardware - just because they feel like it. Disgraceful.