Legacy Sprite Packer eventually put sprites into separated atlas textures.

The texture "5" was put into a separated atlas texture from the other numbers and marked as RGBA32 format instead of ETC2, while generally all the settings are the same.
There's no any platform override settings as well.
Is there any clue on how this can happen?

 

Check if there is any override if you select the "Android" tab in the Texture Inspector. Your screenshots show the default settings only, these are not necessarily what's used on Android.

 

No override was used. The importer will be completely reset by a script.