Just tested and it seems to be an issue because the build is in 1920x1080 (and I managed the scaling badly in Unity)
I have no issue on my 4K 16:9 monitor but in my 1920x1080 I have the same issue
I've modified the settings so that the game is directly in the page now, with that the fullscreen button on the bottom right is working and should fix the issue (I hope for everyone)
Thanks a lot for the feedback, I totally missed this issue!