Message I Phone 6s

Hi, I have an Iphone 6s, iOS Version 11.2 (15C114), 64GB.

At some point, without doing anything (no back ups, no resets) the app Message started to work strangely.

Every time I turned the device off, or put it in Airplane mode, the old messages were put at the beginning of the list (as if I received them recently). The first time it happened, I canceled them thinking on a one time problem. But unfortunately it happened several times every time with older messages.

Apple suggested me to do a back up reset the phone like a new iPhone and then importing the back up. I did it and the problem was still there. So Apple told me the problem was on my datas and suggested me to reset the phone like a new iPhone without importing the back up.

This could be a solution but I would not have my old datas (except what I have on iCloud) and Apps on the phone. Also this procedure doesn't assure me that in 1 year the problem doesn't appear again.

I was wondering if someone has a different solution who can fix the message problem without loosing the back up.

Sincerely, as I paid 700 euros for this phone, I expected that iPhone worked easily without those kind of problems.

Thanks

Massimo