Just wanted to follow-up in case others run across this. I believe the root cause was using a different version of Winlogbeat (7.9.3) than Elasticsearch (7.9.1). At any rate, I reached out to @ManuelF who had a similar issue and he gave me great advice. When I exported the visualizations I needed to also append .keyword to the fieldnames. Once I had done that and re-imported the visualization, I no longer got the "Saved 'field' parameter is now invalid."
(Thank you @flash1293 for your reply - sorry for not getting back faster.)