If you die and instead of respawning you click on the button to go back to the main menu, then reload the save or re-join the server your player character collision box will be significantly smaller.
This allows you to partially phase through walls, which displays some weird black artifacts (I believe these render as if you were looking at them in first person?), and even walk into 1x1 gaps, though you will take suffocation damage.
The bug isn't permanent, it goes away if you reload the world or re-join.
Attached video is a singleplayer save, but the issue also happens in multiplayer, tested by having a friend join me, die, go back to the main menu then re-join, and got the same result.
Thank you for your report!
We're tracking this issue in MCPE-167045, so this ticket is being resolved and linked as a duplicate.
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