instead of using the field to display the URL links that are randomly working, create an expression with Arcade and then use that expression to display the link in the pop-up. Below is a link to a tutorial on how to configure an expression (under the "use expressions" section) in the new map viewer and also a sample of my arcade code that builds an expression for a URL. Hope this helps!
tutorial link: https://doc.arcgis.com/en/arcgis-online/create-maps/configure-pop-ups-mv.htm
sample of arcade code I use to configure a expression for a URL:
'https:'+TextFormatting.ForwardSlash + TextFormatting.ForwardSlash+'www.pa.marion.fl.us'
Amanda Bishop, GISP