This happens especially with styles associated to a workspace.
The styles are correctly stored into the backup zip file along with their descriptors.
When trying to restore the zip file, however:
1. the files associated to the styles on the main folder (e.g.: *.sld) seem to be empty and new weird *.bak.1 files appear instead
2. the "styles" subfolder from a workspace is not restored at all. It looks like it is ignored somehow.
No more clues at this time. It seems related to some callback or additional operation that GeoServer performs now and which is not taken into account by the Resotre logic.