Hi @WillyG,
I just wanted to let you know that there is an existing bug logged for this behaviour which is currently in the review stage. The details of the bug are:
- BUG-000150792: In the new ArcGIS Online Map Viewer, after editing text in a pop-up containing hyperlinks, the hyperlinks are automatically removed.
In the meantime a suggested workaround would be to edit your popups using Map Viewer Classic.
I hope this is helpful.