There are two possibilities.
1. You are using small layer panel thumbnails and the canvas is horizontal in one and vertical in the other (which from the masks it appears to be the case). The vertical canvas thumbnail gives room to show a larger thumbnail icon.
2. The thumbnails are set differently
Go to the Layer Panel . Click at the top right and from the menu choose panel options.
In your first file you have small thumbnails and in the second medium thumbnails
It's possible that due your you're particular screen resolution that PS can't decide which icon to show. I seem to recall some earlier versins of CC that gave me both icons in the same file also.
Hi I modified my answer after replying - see "1" above.
Small thumbnails look different with horizontal/vertical canvases