I played:

Luigi's Mansion Pinball

and found a strange bug where the ball was gone and there was a lot of noise after playing for a while. I assumed that the ball spawned over a bumper and contacted the creator, showing him pictures of when it happened but he couldn't reproduce it.

After opening the browser console (F12 on Google Chrome/Chromium) I finally saw what was going on:

POST https://newgrounds.io/gateway_v3.php net::ERR_BLOCKED_BY_CLIENT newgroundsio.min.js:70

In other words, a script related to newgrounds.io was having trouble and, wouldn't you know it, the uMatrix extension (it's like Adblock on steroids) was blocking it:

So there we go, don't block "newgrounds.io" if you want games to work as well as get medals.

Note that this applies to any game (not just HTML) since they'll try to contact that to get access to Newgrounds new API.