Hello,
I know the first question was about the webmap, but I have the same problem in ArcGIS Pro (2.4.1).
The thing here is... that it seems to be random ?
(The advantage in ArcGIS Pro is that I have a simple workaround :
I don't use a line break in the expression, but a coma. I then use this coma as a line breaker in the "Position" menu :
)
PS : sorry for the french ArcGIS Pro