Lav wrote this in his expanded operation file.
overlay_get_position = 946 # (overlay_get_position, <position>, <overlay_id>)
# Retrieves overlay current position to specified position trigger, using position's X and Y coordinates. Note that the screen size in Warband is (1.00,0.75), further modified by fixed point multiplier.
That means Warband considers you run the game with 4:3 aspect ratio like 1024x768. I play A New Dawn mod which uses some sort of Floris mod as base. Also I use 1280x1024 as screen resolution (5:4 aspect ratio, quite close to the 4:3). At that screen, i haven't seen any issues yet. I guess from that high fps rate, you have a great monitor and using the aspect ratio 16:9 like 1920x1080. That aspect ratio may be the thing that deviates too much from the assumed 4:3 ratio and causing the issue. Use some 4:3 or 5:4 aspect ratio resolution (1024x768 or 1280x1024) and see if the problem still exists.