As it states in the title, any End Crystal in the end will explode automatically.
You can test this by opening any world in 1.21 Pre-Release 1, going to The End and trying to place an End Crystal.
Also, End Crystals generated in the End towers are also destroyed.
Thank you for your report!
We're tracking this issue in MC-272550, so this ticket is being resolved and linked as a duplicate.
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