Safari File Dialog Fails to Show Safari's Open / Choose File Dialog Window fails to show for any upload or select file option. This appears to be universal and the dialog window fails to show regardless of which web app, or site is being used.
A new clean account and Safe Boot exhibit the same behaviour.

Safari: Version 14.0.2 (14610.3.7.1.10)
macOS Mojave 10.14.6 (18G7016)
This is now fixed in the latest Safari update, 14.0.3.
More information here https://macintouch.com/community/index.php?threads/problems-with-safari-14-0-2.2926/
And here: https://mrmacintosh.com/security-updates-2020-001-for-catalina-2020-007-mojave-safari-14-0-2/