Legend heading in Pro not visible in Portal Web Map

[AllenScully2]

Have a service published from Pro (2.9.2) with custom headings for the legend (i.e. not the actual column name):

The simple problem is that these just don't show up in a web map I am using with this service, even though I am keeping the default Pro-generated symbology so users can't tell what those ranges actually represent:

Any thoughts about what I may be missing here, or causing the issue?  I am fairly sure this is not default and doesn't happen on other services published to our Portal.

Thanks - 

Allen