Edit: this seems to be device-specific, using this code as a repro.
When I first enable text input by love.keyboard.setTextInput(true), and I press the "backspace" key on my on-screen keyboard, love.keyboard.keypressed will register that the key was pressed, and fire with "backspace" as the first argument.
Once it has fired for the first time, however, it will no longer fire again, and love.keyboard.isdown("backspace") will always return true. This happens whether you use love.keyboard.setTextInput() to set the text input on and off. The backspace will appear to be held until you exit the program.
My phone's love version is 0.10.2, Android version is 5.1.1, Model is Kyocera E6782, and software version is Verizon's 2.998VZ