Next level doesn't unlock even though I completed the preceding level

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As the title says, the next level (22 in the screenshot below) is still locked although I've accomplished the preceding level (21 in the below screenshot) as shown by the below screenshot:
BTW, I'm on the second stage (Rise and Shine) and this is the first time in seeing this problem happen.
- Why doesn't the next level unlock?
- How do I unlock the next level?
Edit: This happened in the next stage too (When Pigs Fly) and all the stages after it.
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𝗢𝟮 𝗜𝗻𝘀𝗶𝗱𝗲 𝗟𝗶𝗻𝗲: 𝗧𝗵𝗲 𝗡𝗲𝘅𝘁 𝗟𝗲𝘃𝗲𝗹 𝗗𝗼𝘄x
Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
Images: Kelly, Tim Samuel, Artem Podrez, Ono Kosuki