Hello, The new Mastery Challenges for Jezioro Bestii aren't showing in game on the PC version. I've just read a couple of tweets saying the console versions are also having the same issue. A screenshot of the Mastery page has been attached. Hope this helps to address it! Thanks
Update - As of the 4th August 2020, the Mastery Challenges for Jezioro Bestii on the PC version are now playable. Thank you