When holding down the trigger for Use Item/Place Block it will only place blocks continuously in one direction. If you move in any direction besides straight while placing blocks it will stop completely. You have to let go of the trigger and press again to be able to place any blocks any where again.
Linked issues
Comments

Thank you for your report!
We're tracking this issue in MCPE-30610, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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