This is an issue with Minecraft Realms beta for PE & Windows 10. I didn't enter it under the Minecraft Realms category because they only list the PC versions under "Affects Versions". This is intended.

Also your list of iPhone models under "Device" in this category is out of date. I have an iPhone 6s Plus, although it doesn't matter since it's a realms issue and was observed on multiple models of iPhone.
Thanks, we will review these problems. -kaleb418

The issue:

We created a Minecraft PE realm 3 person ($3.99) subscription on 6/14 and started building right away.

On 6/16, my daughter informed me that part of a tower i built was missing. I thought it was a glitch on her iPhone, but on 6/17, I logged into the realm and found it had sliced out about 1/3 of my tower and completely removed it. It removed a slice about 8 blocks north to south, and over 30 blocks (at least) east to west. It removed everything from the ground to the maximum build height in the region.

The removal may have affected the world all the way across from west to east for all I know, I only had built a tower in the one area and had nothing beside it either to the west or east. From north to south, the removal was 8 to 10 blocks deep horizontally and consisted of all blocks vertically from the ground up.

From looking at your Mojang Support twitter account, it appears you had a PE realm outage about the time the deletion happened. I just want you to know that your outage resulted in content destruction. Whatever the restoration process was, there are some issues with it as it's not a complete restoration.

Since there is no way to backup a PE realm on the part of the user, having Mojang's PE realm server remove user added blocks is suboptimal. Be aware that it's happening and please look into it. Right now it's hard to know if building things is a waste of time.