How to reproduce:

1. Open the attached project "case_845762-TextureIssue.zip"
2. Open the "TextureIssue" scene from the Scenes folder
3. Select the "AssetBundleLoader" object
4. Set the "Variant" to "var_a" in the Inspector tab
5. Hit play

Expected behavior: Material and Prefab reference the texture in "textures.var_a"
Actual behavior: Material and Prefab reference the texture in "textures.var_a"

6. Stop play mode
7. Set the "Variant" to "var_b" in the Inspector tab
8. Hit play

Expected behavior: Material and Prefab reference the texture in "textures.var_b"
Actual behavior: Prefab works as expected. Material references the texture in "textures.var_a"

Reproducible: 5.4.2f2, 5.5.0b10, 5.6.0a1
Not reproducible: 5.3.5p2