When using all atlas packing features with polygon packing, which includes attachment rotation by 90, 180 and 270 degrees, this will not be loaded correctly by the spine-unity 3.5 runtime. This is because the old atlas format did not support such rotation.
You could have a quick try exporting the atlas separately from the skeleton, using the skeleton exported from Spine 3.5 (.json or .skel.bytes file) in combination with new atlas .png and .atlas.txt files exported from a recent Spine 3.8 or even 4.0-beta version. Be sure to disabling image rotation in the Atlas export settings, and potentially disable whitespace stripping as well. Unfortunately I'm not familiar with what the exact atlas loading capabilities of spine-unity 3.5 were at that time, but disabling all additional features is your best bet.
