Thanks for sending the email, we received your package, but it is 2.6GB in size (uncompressed about 7GB). In general, never include the Library directory when sending reproduction projects, that alone contributed 5GB of unpacked size on disk, and will be deleted before opening the project anyway. Apart from that, your project is far from being minimal, and it does not compile after opening it in Unity (related to your used thirdparty plugins), thus we can't even enter play mode in your test scene. Also you mention in your email that the problem occurs when using a custom flutter Unity widget on Android, which adds yet another unnecessary layer around it.
What I forgot to mention earlier, there are multiple causes for skin repacking to end up failing with white textures, which are documented in the Note box here:
http://esotericsoftware.com/spine-unity#Combining-Skins
Please check these listed points, perhaps Texture settings for Android are messed up in your problematic textures.
If none of the above resolve your problem, you can try building a really minimal reproduction project this time which does not use any thirdparty plugins, builds directly to Android without any wrapper layers and uses only one or two scripts to reproduce the problem. The reproduction project shall not include a game, you need to strip the logic down to include only the problematic logic, no other lines of code.
