Your problem description suggests a mismatch between project settings and your export settings.
Therefore, please give details
a. Properties of your source video
b. Project preset set by you (manually) or the project automatically to match the properties of your source media
c. What are the details of your export, Publish+Share/then what, then what, and what did you set in the Presets field?
Did you use the default settings for the preset selected or did you customize the preset's export settings under the Advanced Button
of the preset selected?
Please review, consider, and supply the information requested in order for us to give you the best possible reply to resolve your issue.