I usually play on normal difficulty but whenever I'm building nether tunnels I almost always switch the difficulty to peaceful ghasts can't destroy the bridge I'm building. Ever since 1.14.0 however, ghasts seem to spawn in the nether regardless of whether I've set the difficulty peaceful or not. This bug has still not been fixed and/or resolved as of v1.14.20 and affects multiple versions such as Nintendo Switch, PS4 and Mobile.
Thank you for your report!
We're already tracking this issue at MCPE-53383, so I will resolve and link this ticket 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: 📓 Issue Guidelines – 💬 Community Support – 📧 Customer Support – 📖 Game Wiki