(Had to use 1.14.0 as 1.15 does not exist as a tag yet) Mobs can now spawn on tree leaves and stair blocks during night time and low light levels. This issue was fixed a very long time ago, and has been reintroduced with 1.15 Bee Update.
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Auldrick
Thank you for your report!
We're already tracking this issue at MCPE-54241, so I will resolve and link 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.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
Incidentally, 1.14.0 is the current Bedrock release; 1.15 is the corresponding Java release. As the release numbers have now come close together and will soon overlap, we can expect to see more confusion of this kind, so please always go by the release number shown on the main menu.
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Can confirm after creating flat world with leaves that hostile mobs can spawn on top of leaf blocks. And worst of all, no way to easily light them as torches cannot be placed on leaves.