Question Sprites missing from assetbundle variants

I'm trying to use variants for localization but I'm having issues.
Prefab PREFAB1 in assetbundle ab_prefab1 (no variant)
Prefab PREFAB2 in assetbundle ab_prefab2 (no variant)
Sprite A,B,C,D in assetbundle ab_sprite.base (base variant)
Sprite A,B,C,D in assetbundle ab_sprtte.other (other variant)

PREFAB1 has UI.Image which references A,B,C from base variant. everything works fine when I use 'base' variant, but when I use 'other' variant sprite A from 'other' variant is appearing, but B,C are missing. inspector tells target sprite is missing.
PREFAB2 has UI.Image which references D from base variant. it works fine with both variant.

Sprites are not included in sprite atlas. I tested using sprite atlas for both variant, but it didn't work.

I'm using heavily customized AssetbundleManager but baseline is same : load all dependancy first, then instantinate prefab. it works perfectly when I use 'base' variant, or load sprite dynamically.

it's happening all across our project. I'm very confused since not every sprite are missing but only some.
both works fine when I dynamically load sprite, but our enormous project size prohibits me to do so.

this behaivor makes me very confused. I'll rather understand situation much easily if no sprites were working...
Is there any known issue about loading prefab-assigned sprite from variant? or am I doing something wrong?