Nextion screen sending serial data on waveform update

I’ve encountered some unusual behavior on the serial port, and I was hoping someone could explain it.

On page 0, I have a one second timer which updates a value to a waveform, also on page 0. My code is below

add 15,0,OTV.val // send OTV to channel 0

I had the wrong value for the id. It should have been 19 but I wrote 15, which happened to be a text box. Instead of adding a value to the waveform, I was trying to add a value to a text box. The program compiled without errors, but during runtime the serial port would send a \x12\xff\xff once a second, the same frequency as the timer.

I corrected the error and the serial port communication stopped.

Can anyone tell me why this was happening?