Hi. I assume that « glass » refers to the part of the hair that needs transparency and the hair you used is probably made differently The shader most cas item use does not support transparency. If the item needs it, the creator changes the shader to the one that does. It results in the issue known as Z sorting (you do not need to understand that). To fix this issue, people either layer the strands in a specific order (works only with very simple hairstyles ) or split it into the two parts: the one that needs transparency and the one that does not
Anyway, none of it matters. This issue is ts4 specific, blender does not struggle with it, so you can disregard it What you should not have done was enable « remove doubles ». It may remove the inner side on the hair
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