Are you using Portal for ArcGIS 10.6.1 (or newer) as part of an ArcGIS Enterprise setup?
I was experiencing the same behavior in Portal for ArcGIS 10.6.1 and opened a case with Esri support. They finally concluded,
"I do not believe we are going to be finding an answer as to why this is happening. It seems like, from what development has told me, the category selectors ability/stability at 10.6.1 was not very good as they completed a lot of work of the action framework to make it better in 10.7 and ArcGIS Online. At this moment I do not have any further tests to complete and I am out of ideas (and development does not seem to have any either)."
Esri believes it will go away if we upgrade to ArcGIS Enterprise 10.7 or use ArcGIS Online's Operations Dashboard. We have not tried either suggestion.
I hope this helps. Reach out to me if you would like more details on our case.