EXPLANATION:
I noticed this happens when using different menus to export.
When exporting via Asset Actions -> Export the EMode enum is correctly written into the FBX settings.
However, when I:
Open the Animation Sequence -> Top Bar -> Export Asset -> Preview Mesh
Unreal Engine fails to setup the TimeMode setting.
I have submitted a bug report and will update the post if a ticket is created for it.
Hope this helps.