Task history is purging quicker after upgrading to 8.7.3 compared to prior to upgrading.
Clean-up Task Data settings have reverted back to the default values of 200,000 rows for the maximum number of working database rows and summary rows. Depending on the size of the environment and available SQL resources, this is often set to 1 million rows or higher.
Navigate to Settings > All Settings > Notification Server > Task Settings > Clean up Task Data.
Set the maximums to the previously set values or to a higher value as needed. You can also enable the option for the Minimum and Maximum time period to keep this data.
For more information, see the documentation for Cleaning up Task Data