Update-
Contacted Esri support regarding this issue and turns out it is in fact a bug.
Here are the details for those who may also run into this issue-
Bug Number: BUG-000163691 Synopsis: Symbology is missing in the map when exported as a PDF file if using gradient fill symbology. Status: In Review
Additional Notes: The issue is resulting from changing the Extent in Gradient Fill settings to Absolute. The issue does not reproduce if use Relative Extent in the Gradient Fill settings. The issue does not reproduce when exported as Image (JPEG & JPEG 2000).