Interesting, thanks Michael. Actually I hadn't looked at this since we went full enterprise with a federated and hosting sites at 10.6.  I also have Monitor at 10.7.  Were at 10.61 now moving to 10.7 shortly. 

But since you replied here I just went back into the stats page at my current 10.6.1, using Chrome 73.x for the first time in about 18 months and am not experiencing the statistics control check box issue....

I am using Chrome 69.x and the statistics checkboxes still do not work in AGS Manager 10.5.1, so maybe this issue was fixed in a patch for 10.6.1 but not the older 10.5.1.

Is Monitor separate ESRI software that you need to pay for in addition to the base ArcGIS Enterprise (Server) software?

I wouldn't be surprised.  Yes, Monitor is a separate, licences app not included in Enterprise Advanced, Standard, or Basic. But it is worth every penny.