I've been informed by ESRI support that the line spacing of pop-ups in Web App Builder is hard coded. This means that line spacing in web map popups configured using the custom formatting option will not be represented in any web apps that reference it:
The code that defines the line spacing can be over ridden when using Web App Builder Developer Edition, but one of the key benefits to using the AGOL platform is to avoid having to maintain any custom code.
My request is to ensure all facets of the Web Map custom pop-up editor are honoured in the web app builder.
(I reported this as a bug to ESRI UK support CAS-24205-X7W3J0, but was informed it needed to be raised as an idea, so here it is!)