Pixelmon-1.12.2-8.1.2-universal.jar with winrar
navigate to \assets\pixelmon\spawning and look for BetterSpawnerConfig.json an open it with your text editor.
look for (ctrl+f to search) "seesSkyException" and "treeTop" and paste the below snippet at the bottom of each array [ code ]:
"dynamictrees:leaves0",
"dynamictrees:leaves1",
"dynamictreesbop:leaves_flowering",
"dynamictreesbop:leaves0",
"dynamictreesbop:leaves1",
"dynamictreesbop:leaves2",
"dynamictreesbop:leaves3",
"dynamictreesbop:leaves4",
"dynamictreesbop:leaves_palm",
"dynamictreesbop:leaves7",
"dynamictreesphc:leaves0"
and then save and close. this should fix it, it worked for me.