It might be a problem with the game.
The game automatically changes the video resolution when it starts.
"FH4 KNOWN ISSUES [UPDATED: OCTOBER 1ST, 2018: 5:33 PM PT" by FORZA TOTAL.
"Running with an Overlay or On Screen Display via MSI Afterburner, RivaTuner or EVGA Precision XOC can cause the game to crash at launch or during gameplay. "
Please refer to https://forums.forzamotorsport.net/turn ... -2018.aspx
We will also keep an eye on this issue.
Thank you for your report.