Reguarding this full screen bug,for people having this issue,are they all using windows 10.
Specifically the fall creators update has caused issues on iracing with the resizing of the window,they have solved it by turning of the game bar display in windows settings
“One work around for this seems to be to disable the game bar. In the fall creators update, there is no option to turn off game mode like there was in the first creators update. Instead, one way, is in the windows settings, go to gaming, and in the game bar settings, uncheck the option to "show the game bar when I play full screen games Microsoft has verified", and toggle off the game bar. This seems to help. Another fix seems to be to create a shortcut for the sim, and in the compatibility settings, check "disable full screen optimizations". Either of these approaches seem to help.
Worth a try for those experiencing this full screen issue