A bit of an update on this bug. Over the last year or so I’ve been trying different combinations of settings on my test SFOS phones (XA2 standard and, now, Xperia 10 iii) to see if I could narrow down the problem and I think I am getting somewhere.
When all of the above symptoms occur (i.e. the Exchange or Office365 account stops synchronising, and nothing will fix it short of deleting the account and setting it up again from scratch) I have the account set to synchronise email, contacts and calendar.
However, I have found that if you set the account to synchronise email ONLY (i.e. contacts and calendar are unchecked in the sync profile) then the account will continue to synchronise email indefinitely, even when the network changes from wifi to mobile - so, for me, this is major progress. I’ve had email synchronising reliably on my XA2 Plus for two months now without a problem, and on my X10 iii for as long as it takes until the phone needs to be re-booted for other reasons (loss of sound, loss of mobile data in Android, etc).
This synchronisation reliability still (since 4.4) doesn’t survive a re-boot though. If you re-boot the phone then the Exchange/Office365 accounts just stop working completely and you have to delete/re-create them to get things working again - but it is progress.
Furthermore, if you then enable calendar synchronisation then things still appear to work (more, longer term testing required) - but if you enable one-way/two-way contacts synchronisation then this is when the problems all come back again. So maybe the issue could be with the contacts synchronisation, and it is this that is causing the account to fail?