Why is the leather tunic texture failing when I edit it?
Solution 1:
In your resource pack, leather_layer_1.png is 32x32 pixel. In 1.12.2 is should be 64x32.
You can not just change the size of the image as you need to add the arm cover data.
Here is your file, resized with added arm and back sections. Don't download this image as it has been enlarged so it can be seen here:
Here it is to scale so you can download it:
You can modify the colored sections to add your own texture. Make sure they are grey scaled when done. I used color to make them stand out but if you leave color, the armor will not change color when dyed.