why fgetl read '\n'?

Hi, all. I'd like to ask for advice.

What I'm laying is to show up the string data transmitted from AVR using USART(Serial communication). My AVR is sending three ascii data generated from sensors and '\n'. I call this total four data as 'data group'. Let me give you an example as below.

0\n

-5\n

10\n

\n

20\n

10\n

-1\n

\n

... and so on

So, three data would be transmitted and fourth data '\n' will sort of report the end of the current data group. I use 'fgetl'(fgetl - Read line of text from instrument and discard terminator) and expected as below, repeating N_mat times. (Please refer to the code attached!)

roll = 0 -> roll = 20 ->... and so on

pitch = -5 -> pitch = 10 ->... and so on

pressure = 10 -> pressure = -1 ->... and so on

N_mat = 50;
t_pause = 0.001;
bt = Bluetooth('JCNET-BT-7827',1);
fopen(bt);

for i = 1:1:N_mat    
        % remove data from input buffer
        flushinput(bt);
        % find the point for starting data group
        LF = fgetl(bt);
        tic(); pause(t_pause); toc();
        while ~isempty(LF) 
            flushinput(bt);
            LF = fgetl(bt);
            tic(); pause(t_pause); toc();
        end
        % get the data in order if the starting point is found
        roll = fgetl(bt);
        tic(); pause(t_pause); toc();
        pitch = fgetl(bt);
        tic(); pause(t_pause); toc();
        pressure = fgetl(bt);
        tic(); pause(t_pause); toc();
end

Yes, it works as I expected. But roll, or pitch or pressure occasionally get ' ', which means nothing(0x0 char). I have no idea why this happen. Anyone can give me a help?