Potions on bedrock ignore armor AND enchantments (In java they only partially ignore armor) because of this harming arrows almost oneshot no matter the gear the target is wearing ( With multishot it will always oneshot) Posion will always put you at half a heart and the withers wither effect will always kill.
Related issues
Comments

Thank you for your report!
We're tracking this issue in MCPE-40651, 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
-- I am a bot. This action was performed automatically! The ticket was resolved by one of our moderators, and I left this message to give more information to you.