Okay, I just confirmed that this is happening.
I completed a puzzle and was then given puzzle #eKC1R https://lichess.org/training/eKC1R
I never attempted that puzzle. The tab with it displaying is still open. I'm pretty sure that I refreshed that page at the time (under an hour ago) and it kept displaying the same puzzle.
Now if I go to Puzzles I am shown puzzle #Z1F7D https://lichess.org/training/Z1F7D
I also just refreshed the original page to confirm that it will change to #Z1F7D too (it did).
Puzzle #eKC1R does not appear in my puzzle history either as a success or a failure, neither on the puzzle page itself or on the Puzzle History page.
edit: I have mentally noted that this appears more likely to happen with puzzles that have a low play count, but this could be a red herring. edit2: definitely a red herring, just happened with a higher play count
Okay, I just confirmed that this is happening.
I completed a puzzle and was then given puzzle #eKC1R https://lichess.org/training/eKC1R
I never attempted that puzzle. The tab with it displaying is still open. I'm pretty sure that I refreshed that page at the time (under an hour ago) and it kept displaying the same puzzle.
Now if I go to Puzzles I am shown puzzle #Z1F7D https://lichess.org/training/Z1F7D
I also just refreshed the original page to confirm that it will change to #Z1F7D too (it did).
Puzzle #eKC1R does not appear in my puzzle history either as a success or a failure, neither on the puzzle page itself or on the Puzzle History page.
edit: I have mentally noted that this appears more likely to happen with puzzles that have a low play count, but this could be a red herring. edit2: definitely a red herring, just happened with a higher play count