when leaves are against a wall or mountain you can see through it

What texture pack & mods are you using?

texture pack wise should only effect selective things such as 3d crops, 3d sun and moon, better glow berries, better vines. otherwise everything else is vanilla.
Mods however im using quite a Bit, but world gen mods are oh the biomes you'll go, Promanade, JJthunder hills, better nether,better end and few structure mods such as When dungeons arise, better villages, explore the world.
in total im using 231 mods.

this occurs with vanilla leaves/blocks as well, whenever its placed on the floor or side of the wall i can see though it

What are your render settings.... wait your on 1.18.2?

im on 1.19 and my render settings for Cull leaves is default, render distance is 15, 30 for screenshots
leaves quality is fast, clouds are fast.

When leaves are set to fast, they shouldn't be invisible.

the leaves themselves arnt invisible its the face connected to the floor or wall that becomes transparent allowing to see through the underworld caves or through someones house

Yes although when leaves are set to fast, they are usually opaque.

I will need to check a couple settings

texture pack wise should only effect selective things such as 3d crops, 3d sun and moon, better glow berries, better vines. otherwise everything else is vanilla. Mods however im using quite a Bit, but world gen mods are oh the biomes you'll go, Promanade, JJthunder hills, better nether,better end and few structure mods such as When dungeons arise, better villages, explore the world. in total im using 231 mods.

this occurs with vanilla leaves/blocks as well, whenever its placed on the floor or side of the wall i can see though it

its not on the problem, but dont use fast leaves with betterx mods, it breaks resources of the mod group ;)