The reactions on the catalyst surface are reversible all it is doing is weakening the bonds. Both cis and trans will react with it.
What if instead of the mechanism shown, the second hydrogen was not picked up? What if first hydrogen is lost and the alkene reforms? Can you draw the mechanism for that?
Ok I drew it out for myself and it makes sense to me! Here's what I got (textually
1. After the hydrogen attaches to the first carbon, it becomes tetrahedral and allows free rotation. The two R groups can now be repelled.
2. The second hydrogen does not react. Double bond reforms and double bond is now trans. Hydrogen probably becomes H2 or reattaches onto the metal catalyst.
I drew it out on paper so I hope you're not bothered by a lack of image.
If i'm right, thanks very much! I understand it well now.