This is only happening on Server, not on Desktop.
Have tested on 10.5.2 and 2018.1.
I have a dashboard that has a dual axis map of the US with county level detail and some mapped locations of cities for the second axis. Below the map are two other worksheets that are filtered by the selections on the map. When I select a few counties with the rectangle selection tool and then click on the scroll bar without moving the scroll bar down, the selections are cleared. If I move the scroll bar instead of just a single click the selections stay. If there is enough data in one of the worksheets below the map to create a second scroll bar and I scroll down to the worksheet and then move the scroll bar on that sheet it seems to keep everything selected as expected.
I created a basic version with the World Indicators sample data and I'm able to reproduce the issue on this once I publish it to our server.
Select some data points
Click on the scroll bar and the selection clears
If you drag the scroll bar down the selection stays
Just found out it also happens with just the map worksheet as long as there is a scroll bar on it.
Thank you for sharing this information !
I indeed reproduced it too, and checked internally.
This is currently being investigated though and will hopefully be fixed soon.