I strongly believe this is a bug in the services. I used reports with tooltips this morning without any issues, but now (14pm) I get the same experience as you describe - the tooltip popup and I can see data changing, but it's only visible less than a second and then it disapear again.
We have this exact same problem in our line charts reported by our users this morning. We are located in Norway. This is happening on our report page tooltips, where they display for an instant and then dissappear. It is working in Power BI desktop but not in Power BI service.
I've a several Power BI-reports that I use the superb popup functionality in. So when you hover over a object in a visual component you get more detail information about the object for example. My end users have used these report for a couple of months now but in the last days they have experienced problems with the popup window. When they hover over an object the popup shows and disappers within a second. If they hover again over the object it shows and stays. But only if you do this on the same object again. If they moves to the next object it only shows up for a second again. A suspition is that the information in the popup have not finished loaded within some time limit and the popup closes. This is a new behavior in Power BI Service. I have tested IE , Edge and Chrome. Same behaviour.
The Power BI Service Reports gets its data from a on-premise datagateway and a SSAS tabular model on-prem.
It works fine in Power BI Desktop.
Seems like the threshold for the popup to close should be raised???