Ok... Let's see.
Hello over here I've been facing this same issue, some more or some fewer details, depending on day, build version of Windows 10 Mobile, or other facts. After a long while, thinking on which one of all the threads about this issue would be the most accurate to post my experience:
http://forums.windowscentral.com/wi...review/395689-sms-syncing-fix-10586-11-a.html
http://forums.windowscentral.com/windows-10-mobile/395146-2.htm
http://forums.windowscentral.com/mi...a-950-dual-sim-how-restore-text-messages.html
...I decided it would be the one with the most recent answer.
I have a Lumia 1020 with, right now, the last build of Windows 10 Mobile available via the Insider Preview Ring (you shall know that the x20 spectrum is excluded via the Fast one). Since october 2015 (month which matches with a hardware failure on my handset; failure which reparation extended during two months and a half, time during which the phone was obviously off, and that ended on forcing me to buy a new 1020) everytime I have hard-resetted it throughout the successive build releases, I got the text messages restored up to that month and year.
On the paragraph above I've said I face the issue with "some more or some fewer details depending on day" due to some actions that seem to show (but I haven't been able to confirm it; what's more, on paragraphs below you'll see I'm rather close to deny it) the texts ARE indeed being backed-up. If I e.g. delete a conversation (an entire one... see here
http://forums.windowscentral.com/mi...696-old-text-messages-return.html#post3445315), a couple of minutes later, it appears again. It only happens, surprisingly, with conversations subsequent to 10-2015. If I delete it a second time, it seems to "really sync" and it doesn't reappear.
I've done that rare somersault of switching on the "Contacts" category on Outlook.com in order to see the folder at issue on the left pane, and I've met even rarer facts: firstly, the most of but NOT ALL messages previous to 10-2015 are displayed; secondly, since my native language is spanish, the names of contacts which have accents (?, ?) are displayed as ~Aa or ~Ee, etc.
The third rare fact takes me to talk about the Insider Preview Program on PC (see here
http://forums.windowscentral.com/wi...syncing-windows-10-desktop-messaging-app.html). I'm not enrolled to it, but I'd like to know whether enrolling would help to solve all this mess. Since the adding of the "History" tab to Contacts App (on both PC and handset W10 versions), on PC, casually (causally?) I see the text messages and calls up to... yes! you've got it! Up to october 2015. Due to the Cortana functionalities regarding to send SMS through the phone included on Redstone, that's where I point when I ask if getting that build, on PC, would benefit me.