ComputerCraft mining turtle doesn't stop diging down

Im having an issue with my mining turtle code. Here's my code

function mine ()
    done = false

    while not done do
        print("Moving to desired starting height...")

        while not (TURTLE_POSITION.y == heightToStart) do
            turtle.digDown()
            turtle.down()
        end

        print("Starting height reached. Filtering Items...")

        filterItems()

        print("Filtered Items. Starting to mine cuboid...")

        done = true
    end
end

The problem is that the turtle keeps diging down no matter what. It doesn't stop until it reaches bedrock level.

EDIT: The code for the TURTLE_POSITION

function locateTurtle ()
    print("Attempting to get location")
    location = vector.new(gps.locate(5))

    if not location.x then
        print("Couldn't get location")
        error()
    end

    print("Found location")

    return location
end

refuelTurtle(calculateFuelUsage(cuboidX, cuboidY, cuboidZ))

local TURTLE_POSITION = locateTurtle()

So by the comments I received it was a simple problem: I forgot to update the turtle's position variable

My code now

function mine ()
    done = false

    while not done do
        print("Moving to desired starting height...")

        while not (TURTLE_POSITION.y == heightToStart) do
            turtle.digDown()
            turtle.down()
            TURTLE_POSITION = locateTurtle()
        end

        print("Starting height reached. Filtering Items...")

        filterItems()

        print("Filtered Items. Starting to mine cuboid...")

        done = true
    end
end

function locateTurtle ()
    print("Attempting to get location")
    location = vector.new(gps.locate(5))

    if not location.x then
        print("Couldn't get location")
        error()
    end

    print("Found location")

    return location
end