Bridge 2021 does not generate JPEG thumbnails for NEF files.
Deepak Singla commented
Thanks for your feedback.
Could you share the following details about the issue:
Is the issue specific to particular files/format?
Please share the sample file/files on which you are observing the issue at firstname.lastname@example.org
Arthur Jacoby commented
The thumbnails for NEF files are displayed in 2020 Bridge