Hi everyone - Just like bgro detailed above, I have the same issue. I did a bunch of research online and saw multiple posts on here as well as on the Apple support forums but I haven't seen anyone post any resolutions. Apple currently has my Icloud account in troubleshooting mode so they can look for the issue for about a week but I have not heard back from them.
A couple things you can try are restore to your last Icloud backup created by your IPhone 6 but keep in mind you will lose any data including text messages that were created after that restore date. I did that but lost three weeks worth of text messages and voice message history including any new apps I downloaded (just had to re-download.) While I was disappointed to lose so much information I was glad I was able to recover the majority of my text history.
Here is the only solution I have found that I have not seen posted anywhere that has worked for me to recover my lost text messages. I'm not sure if I can mention other companies software but if you google iphonebackupextractor by Reincubate they have a program that they recently updated which is able to download your Icloud backup for the 6s. You can download your entire backup or select different categories of information to download. The process was very easy and their support team is very quick at replying to questions thru emails. In fact when I initially downloaded their program about 2 weeks ago their software was not able to extract the Icloud backup for the 6s and their support team notified me of this issue and said their software developers were working on an update to address this issue and gave me an option to refund my money or wait a week for the update to roll-out. Within one week their support team emailed me and said the update was available and I downloaded the latest version and it worked great.
They have an option once you have downloaded your sms data to convert it to an html document and save it. This option works really well and the document looks like an actual text message conversation.
On their website they have a blog and also a youtube video which shows steps on how to take the recovered sms data one you have downloaded and how to get it back onto your phone. I have not tried this myself as I'm in a catch 22 where over the past three weeks I have created a bunch of new messages that I will lose if I decide to restore my old sms data backup.
I hope this helps anyone that has lost their messages