Seed: -1600251074
Coordinates: 120, -50, 478
Placing a block on the surface of the lava immediately turns a large portion of the surface to obsidian, but not the entire ocean.
In the picture (apologies for the quality) I had already placed a block elsewhere.
(Picture taken with Parallax shaders to improve underwater visibility)
Thank you for your report!
We're tracking this issue in MCPE-128022, 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