The issue you're experiencing is due to the "Nonessential" data option not being checked during the export of the .skel file. This results in the loss of internal edges for skinned mesh attachments, which can cause transparency issues and other visual discrepancies when importing the file back into Spine. The nonessential data includes additional information necessary for editing in Spine, such as internal mesh edges, which are not required for runtime but are crucial for re-importing and editing. To resolve this, you would need the original .spine project file or a re-export of the .skel file with the "Nonessential" option checked.

NuriNuri The reason for the message about the loss of inner edges is as explained by Spinebot. As for the issue of some of the images becoming transparent, it is not clear from this image alone what the cause is, but it appears that some images are distorted, so perhaps the import scale is incorrect. You may find our following blog post on importing skeletal data helpful:
https://esotericsoftware.com/blog/Importing-skeleton-data