[Update] No longer an issue apparently? Just retried for the 8th time and its fine? So not a consistently reproducible issue.
A world that has used the SphaxPure BD Craft Resource Pack and HD UI and forced other players to download was uploaded to a Minecraft Realm. After re-downloading it and removing these, all Players inventories (XP, Items, Armor, etc...) was lost but map changes such as buildings and changes to the environment stay in effect.
Effects present when doing this on both Xbox One and PC - Windows 10.
Related issues
Comments
Auldrick
Okay. Restating the events, I had downloaded the world and attempted this on both Xbox One and Windows 10. I downloaded the world at separate times of at least ten minutes. On both systems, upon starting the world, when the problem was present, all player characters had lost all inventory items and EXP progress but all physical changes to terrain/buildings stayed in effect. The only issue was all of the Inventories were lost. I then attempted again without removing the resource pack and all individual player progress stayed in the world. The problem only occurred when I disable/removed the SphaxPure BD Craft Resource Pack and UI. However, after attempting multiple times, it suddenly didn't affect any player progress and I have yet to try again. I will test again soon to see if the problem persists.
Auldrick
I would be cautious about testing again, unless it's in a way that doesn't affect your players. There are a number of similar recent reports about players' armor, inventory, and XP being reset when they switch between Realms and offline or local multiplayer. Apparently, the problem happens randomly, so you might see it return.
Auldrick
Try restarting the game. This is a duplicate of MCPE-29863, a known bug that should be corrected soon.
Are you saying the map changes were reverted, and/or the players got their stuff back? It may have resolved for you, but it might still be an issue we should fix for somebody else.