I just came here to report this bug too, seems @NateDhaliwal beat me to it 
There are two scenarios.
On mobile, click on reactions and the summary table is only visible for 1 second before closing.
On desktop, it stays open - but only if your cursor stays hovering over it. If you click on the reactions and move your mouse away it also closes in 1 second, like it does on mobile.
This is only occurring after I updated my Discourse this morning and like @NateDhaliwal I can also replicate here on meta.
I’m currently running 3.5.0.beta1-dev (cbe9a9f33b).