Hi @Katie_Godding ,
Great thanks for reporting this issue !
We also detected this issue earlier, and I'm glad to share Legend in Export window has been significantly enhanced in V2.1. The legend in Export window mirrors to the content in legend tab (on the right side of Data tab in Table of Contents panel ).
For example, the enhancement in V2.1 is as following screenshot, comparing with that in V1.18.
V2.1 is released recently and now it is available, please find new features and highlights here and feel free to download it to have a try .
https://doc.arcgis.com/en/arcgis-earth/get-started/what-s-new.htm
Please do not hesitate to contact us if you have further questions.
Thanks.
Sharon