Can see private tweets via keyword searches on tweetdeck

How it was found

I'm not the best at finding "why" this happened, but I am pretty sure it's not supposed to. I was keyword searching "protonmail invite" and this came up even though the user was not on public nor did i follow him. I go to twitter.com and search for the exact same tweet, nothing comes up of his tweet. I logged into a different tweetdeck account - I can still see the tweet by searching for it, the user is on private and I do not follow him.