Well, I use an Android Tablet included with a pen, there's no trick needed. I just hover my pen over those reactions and simply I could see who have reacted.

And I know the trick to see for users having a smartphone,

That is,

Click and hold on a reaction for 2 - 3 seconds.

Then the reacted people will appear(for the certain reaction)

Are you using google chrome? Then it will be succesful.

In this case, I've found that the reactions will appear when you do that twice.

When you do it for the second time, drop down list appear.(In both times it appear) like this. Then click anywhere around that list as shown below.( Red dotted areas)

Then you'll be able to see the people.