Can't say exactly what criteria is used to set the export frame rate, but auto detect isn't always the choice or may not even be listed.
Just click the caret and change it to the desired fps. If you forget, the project won't be ruined as you can always export again.
Make your concern known here.
Save Project File AS routinely, giving it a unique # or name, so as not to overwrite prior saves. Using Save Project Files (no AS) will overwrite. From the HOME tab, that choice is available by clicking the caret to the right of the Save Project button on the toolbar.