Please disregard. I found the answer on Community from 2021:
https://community.smartsheet.com/discussion/77805/copying-rows-and-the-display-of-gantt-chart
This background can happen when the rows in their entirety have been colored, either manually by using the paint can icon,
Or through a Conditional Formatting rule. Instead of selecting "white" to clear out cell background color, select the row (by clicking on the row number) then choose "Automatic":
Or, change your Conditional Formatting rule to be "No Background" instead of "White". (See: Apply Formatting Automatically with Conditional Formatting Rules.)
Best regards,
Tali