The default value does not appear in the web map pop-up in ArcGIS Enterprise 10.9 and 10.9.1 when the default value is set and 'Allow NULL' is unchecked in the attribute table in ArcGIS Pro.

Workaround

1. Open the same ArcGIS Pro project.
2. Click the Edit tab.
3. In the Features group, select the Feature Template icon in the bottom-right corner.
4. In the Manage Templates pane, select Properties > Attributes.
5. Edit the default value text to a temporary value.
6. Click OK.
7. Repeat the workflow and edit the default value again to display the original text and click OK.
8. Overwrite the feature layer.

Adding a new feature in the web map displays the default value.

Steps to Reproduce