In ArcGIS Online, when adding a map or feature service published to ArcGIS Server using ArcGIS Pro 3.2 or later, HTML formatting in the item description is not honored in ArcGIS Online, resulting in a plain text display without the intended formatting.
This issue is not reproducible in-house anymore. It may have been fixed with the recent upgrade to the item details page. If the issue occurs, contact Esri Support Services.