As far as I know this is the only report of such an issue.
The only reason for this I can think of is the update from Qt 6.3 to Qt 6.4. Qt Design Studio 3.9 now uses Qt 6.4, but there is as mentioned no known issue related to this.
The changes to the welcome page itself were quite minimal and should not be triggered on startup.
Did disabling the Welcome plugin solve this issue?
Maybe this is related to the download of the latest example thumbnails. You can disable this by adding this to your qtdesignstudio.ini file:
QML/Designer/EnableWelcomePageDownload= false