Adding a WMS Service as a content item in ArcGIS Online results in it not being visible in a web map (this behavior is also reproduced in ArcGIS Enterprise portal 11.1 and 11.2).

Informações Adicionais

Solução Provisória

• Within the Map Viewer, use the 'add layer' drop down option. 
• Add layer from URL.
• Paste the URL, and select WMS. 
• Select any required sub-layers. 
• Save.
• Having added the WMS layer this way, it will now be visible in the Map Viewer.

Etapas para Reproduzir