The Google cloud storage (in S3 compatibility mode) caches in default publicly readable objects , e.g. .cvmfswhitelist. This means that e.g. executing mkfs twice in same repository fails, because first created .cvmfswhitelist is always returned.
The caching behaviour can be changed e.g. by adding "Cache-Control: no-cache" header  when uploading these mutable objects. For the immutable files it is probably best to keep the default caching behaviour, defined by each S3 cloud storage, and not to specify explicitly any cache directive. In practise new request type "s3fanout::JobInfo::kReqPutNoCache" (in addition to s3fanout::JobInfo::kReqPut) should be defined, which is used when uploading mutable objects.