This has multiple issues and they all say resolved. It is not resolved, as nothing has been fixed.
2nd player and 3rd lose all progress and inventory even when saved and closed properly.
When game is restarted, players start back at spawn point as if they are new spawns.
Related issues
Comments

Thank you for your report!
We're tracking this issue in MCPE-55815, so this ticket is being resolved and linked as a duplicate. Please note that that does not mean the bug is fixed, it means that the problem has already been reported. Please watch the main ticket for any updates.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
Quick Links:
📓 Bug Tracker Guidelines – 💬 Community Support – 📧 Mojang Support
📓 Project Summary – ✍️ Feedback and Suggestions – 📖 Game Wiki