I don't know what happened but seems the smart leaves resource pack didn't do anything wrong to the other better leaves resource pack. But it happened to my pack and I don't know what is wrong with it.

Pic --> https://imgur.com/a/KJmHVUx
My Personal pack link: https://www.mediafire.com/folder/n2apmskuib8bm/My_Pack

Upon further investigation it seems like your pack is broken. There are literally references of a missing texture in the leaves.json file of your pack, I think it'd also have the same issue without the Smart Leaves pack.

Well, that's weird because it doesn't happen without the Smart Leaves pack.
Anyway, Thank you for your advice :)

Also, It happened only with one model of my leaves resource pack. I tried to fix it But failed.

Seems weird, but the smart leaves pack isn't meant to be used with better leaves packs anyway, since it just replaces the model of the vanilla leaves. You can take a look at the pack here, it's literally just one single file. Maybe that helps you find out what's wrong with your pack.

Upon further investigation it seems like your pack is broken. There are literally references of a missing texture in the leaves.json file of your pack, I think it'd also have the same issue without the Smart Leaves pack.