According to user reports, the Windows 10 SMS backup and restore system is still unreliable in build 10586.63. The reason might be a change in back-end architecture which provokes a loss of saved messages.
Sirapian has tested the Windows 10 Mobile SMS backup and restore feature on several devices such as Lumia 535, Lumia 640 and a brand new Lumia 950 with various builds. In his tests he found incidence that Microsoft might have changed the message format which then causes migration problems.
“But since Dec, a bit before 10586.29 was out, I've noticed AGAIN problems in syncing, on my brand new 950, but also between 640 (which was my main device) and my secondary 535 (which had “only” one week to catch up). Only some messages are restored, there are gaps (and always the same), Skype messages are not restored (when they used to be in November)”
As his tests with the Windows 10 desktop messaging app indicate, backup copies saved on Microsoft´s might be faulty or not compatible, provoking crashes of the app. The problem seems to persist even on the latest Windows 10 Mobile build 10586.63, where only some previously saved message are getting restored.
“Now to add some confusion, I've tested Messaging on PC (W10 “retail”, 1511 update, and on the latest Insider build for PC 11082). Skype messages will show (unlike on mobile). Now try that at home : use the search function and search for a single letter (like e) if you're on 1511, or a word if on RS1 Insider build. You'll see matching SMS!!! Clicking on a message will crash the app; however, it show SMS which are in the cloud but which are never restored. Why ?!
G. I've just upgraded my 640 from 10586.36 to .63, hard reset the device. Messages were restored pretty quickly, but I have more or less a mirror copy of the incomplete restore I have on my main device (950). And even more incomplete, as some messages from late December are missing. I mean over a 5-6 texts conversations, one may be missing.”
Also other users report that some text messages are not syncing, like here in this other thread. While in some cases a hard reset or might get the restore feature working, this is not the case for everybody.
Microsoft´s contact+message backup app which allows to transfer messages to other devices in VCF and XML format might offer a workaround. But this is a complicated solution for any user who wants to just upgrade to a final version of Windows 10 Mobile on one device.
Do you have similar issues with your Windows 10 Mobile build. Please leave a comment, reporting your build number and the device you are running.
Source: Microsoft Answers