It has been 5 days since I did a full clean reinstall of MSFS 2024 and with 50+ startups not once has the start button been missing whereas it was happening more often than not beforehand.
I therefore strongly recommend a full clean reinstall of MSFS 2024 and by clean I mean uninstall it first then go into your user’s roaming folder and delete any remnant of Microsoft Flight Simulator 2024 in there. For Steam, this folder will be %appdata%\Microsoft Flight Simulator 2024 and for MS Store this folder is %appdata%\Packages\Microsoft.Limitless_8wekyb3d8bbwe\LocalCache. Then reinstall.