Steps to reproduce:
1st method:
Switch to the D-Pad & tap to interact control mode
Create/enter a world
Place down and ride a boat (with chest)
Go to Settings -> Touch and enable Customize Controls (Experimental)
Go to Customize controls
Press Done
After resuming the game exit the boat by pressing the Leave Boat button
Attempt to move your camera around with 1 finger
2nd method:
Switch to the D-Pad & tap to interact control mode
Create/enter a world
Place down and ride a boat (with chest)
Open and close your inventory
Exit the boat by pressing the Leave Boat button
Attempt to move your camera around with 1 finger
Observed result:
You can't move your camera around with 1 finger until you interact with the screen with 2 fingers.
Expected result:
You can move your camera around with 1 finger normally.
Can confirm in 1.21.2