When viewing the settings history for a Keyword or Creative, you may find that the Custom Parameters you've recently edited have been removed by 'System'.
What Causes The Issue?
When custom parameters are overwritten, it usually means that the original change was not pushed to the Publisher. Therefore, when the platform does its nightly sync, blanks values are pulled from the Publisher and overwrite the values found in the platform.
How To Fix The Issue
If you're experiencing this issue try downloading a report from the platform that includes the columns: Account, Campaign, Group, Custom Parameters, and Landing Page (for Keywords, also include Match Type and Status).
Next, upload this report as a Campaign Bulk Upload (or Keyword bulk upload; whichever applies). The URL Builder will be triggered and the Custom Parameters will be built. This will create an operation in the To Be Sent status in the Activity Log. Go ahead and push these changes to the Publisher.