When sneaking and activating a trapdoor it prevents you from not sneaking. In bedrock edition it should just go through you and you should be able to stand up like normal. (In Java it makes you enter crawl mode).
Steps to reproduce:
Crouch
Stand under and activate a trap door
Thank you for your report!
However, this issue has been closed as Working as Intended.
Not being able to stand up while under an opened trapdoor is a consistent behavior between Bedrock Edition and Java Edition.
If you need help or have a suggestion you might like to follow a link below.
Quick Links:
📓 Issue Guidelines – 💬 Mojang Support – 📓 Project Summary – 📧 Suggestions – 📖 Minecraft Wiki