Applying a fortune II book to a diamond pickaxe has no change in drop rate of diamonds, coal, or any other items. I have attempted multiple mines and not getting any bonus drops.
The book was found in a strong hold chest if that has any bearing on the enchant issue. This was applied and tested in a survival map seed 1769959536, strong hold was located at 146, Y, 1426 with book in library chests.
Thank you for your report!
We're actually already tracking this issue at MCPE-67169, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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