In my biome trees, the leaves fall off spontaneously.

I think there are different block IDs for leaves like there are for logs. One of them shouldnt disappear. But I dont know how you can change the leaves to the other ID (if my theory is right)

ok I did some research and found out what I was looking for:

Oak leaves have the block ID 18. (Other leaves e.g. 18:1, 18:2, ...)

If you want oak leaves not to disappear you need to us 18:8 (command: /give <name> 18:8)

I dont know if you can do this with your own leaves. If mod blocks have ID too you can maybe try something like:

[your leaves ID]:8 (/give <name> d:8)      d=your leaves id

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