*A temporary fix has been posted: https://bugs.mojang.com/browse/REALMS-2168?focusedCommentId=580295&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-580295**  Follow the bug at REALMS-7168**

Leaving my description for continuity:

By looking at my realms backups I can see that sometime between 21:42 on 29/08/19 and 21:22 on 30/08/19 Minecraft realms reports it is on version 1.12.1 (screenshot attached) whilst the highest version available to download from Windows store and Google play is 1.12.0. This obviously causes an issue when you download a copy of the realm. Download works fine but when you attempt to open the world you get the message shown in screenshot #2. 

Downloading the latest backup that is on 1.12.0 in the screenshot works perfectly fine as it should.