http://buildbot.askmonty.org/buildbot/builders/winx64-packages/builds/26370/steps/test/logs/stdio , shows that the file t.ibd is not copied by copy-back, thus it is not in the backup.
A theory :
An empty .ibd is not getting copied into backup, because it is not getting loaded by xb_load_single_table_tablespace . There is a test small_ibd.test, that actually checks that.
Perhaps that does not work well anymore, and the file should be loaded and or copied or something like that.