Strange negative number below battery on watch face.

Hi - can't anyone help me with what this strange negative number is below the battery % on my watch face. My installed watch face is called "Simple TDB" by TomekCz. This is a forerunner 735xt.

It never used to be there, and it's suddenly appeared. I can't recall what (if anything) I did when it first appeared.

I've copied an image in the below link. I' referring to the figure that says -01.17.