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Met the same issue in Unity 2019.4.18f1 for my WebGL build. Random characters missing sometimes. In the editor it does not happen at all.
I believe WebGL 1.0 also support transparent images. I've also run into red question mark when loading textures from web, it was either because...
I met the same issue in Unity 2019.4.7f1 ("Cannot mark assets and scenes in one AssetBundle.") and solved it with @lmbarns's solution. Thanks a lot!