When placing a rail (or redstone) on a hopper, even when crouched, the item places, then disappears, and the hopper opens. It happens every time. This also happens when trying to connect a hopper to a chest (the hopper does not place and the chest opens instead, even when crouching).
Thank you for your report!
We're tracking this issue in MCPE-168280, 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