example file:
D:\Steam\steamapps\common\GarrysMod\garrysmod\addons\customizable_weaponry_2.0_349050451\lua\entities\cw_attpack_ar15_stocks2\shared:
=========================
ENT.Type = "anim"
ENT.Base = "cw_attpack_base"
ENT.PrintName = "AR-15 - stocks package"
ENT.PackageText = "AR-15 Stocks"
ENT.Category = "CW 2.0 Attachments"
ENT.Author = "Spy"
ENT.Spawnable = true
ENT.AdminSpawnable = true

ENT.attachments = {"bg_ar15sturdystock", "bg_ar15heavystock"}
=============================
ttt_cw20conversionmod_attachmentstyle set to 3

ttt_cw20conversionmod_attachmentstyle 1 works fine

I don't see ENT.Custom = true anywhere in this file

dumbass mode, i copied the wrong file, my bad.
================
ENT.Type = "anim"
ENT.Base = "cw_attpack_base"
ENT.PrintName = "AR-15 - stocks package"
ENT.PackageText = "AR-15 Stocks"
ENT.Category = "CW 2.0 Attachments"
ENT.Author = "Spy"
ENT.Spawnable = true
ENT.AdminSpawnable = true
ENT.Custom = true

ENT.attachments = {"bg_ar15sturdystock", "bg_ar15heavystock"}
====================
this would be it

https://gyazo.com/18a0df0fb02c0493486fb7d0da38847a
https://gyazo.com/36a7b401ffa3afa29ef89cfe18890c13

screenshots(custom attpacks boxed in red)

Everything looks good on my end, I don't know what the problem could be. It may be the addon itself. It's possible an update broke it.

A second option is just editing the base packs and it keeping it on mode 1.

https://gyazo.com/04009fbcfe2b2b1969e91a46b0919bc7
https://gyazo.com/ba42b15c9d3bb03661f813b5390eddf5
https://gyazo.com/bc2b66c67ac6647221506c1983b2b4b5
https://gyazo.com/244bf0e754cdd00031787f2e17e64226
https://gyazo.com/85219c9405c4ad042eb1e37da4a359bf
https://gyazo.com/578ff5a0f961caa64283a6c64e01107d

in game screenshots

did changelevel after changing attachmentstyle.

NOTE: the same thing happens even with all nonrelated mods disabled.

may I ask how did you set up your files so i can try to replicate it the way you do?

oh well, modifying the attpacks and setting attachmentstyle to 1 works. thanks for the help