I checked out a version before any of the commits in #82 and found it was still problematic, so this problem predates that work. Tagging @pixelzoom so he is aware.
Known problem, reported in https://github.com/phetsims/sun/issues/618, not specific to RewardDialog, and probably related to the work that @jonathanolson did on factoring out PopupablePanel.
Discovered in https://github.com/phetsims/vegas/issues/82, when testing the reward node in the vegas harness, pressing the buttons in the dialog leads to a broken state:
I checked out a version before any of the commits in #82 and found it was still problematic, so this problem predates that work. Tagging @pixelzoom so he is aware.
@ariel-phet can you please prioritize and assign?