Heyaa! blender help!<3
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It’s hard to tell, but the first image looks like your mesh normals is inverted in blender. Possibly due to negative scaling and can cause your mesh to be invisible unless backface culling is enabled.
Try checking face orientation on the top right of the viewport(2 circles overlapping symbol) under geometry. If it’s red, you need to flip them - edit mode, mesh, normals, flip while selecting affected parts. Your mesh and armature transforms should also be positive 1 in blender.
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When exporting an fbx from blender, Make sure you select FBX all under Transform !!
The first image is just your outlines being messed up since this option wasn't selected on export. It used to happen to me all the time :3