Under the S3 asset report a Storage in GB (Giga Bytes) field is displayed. This can occasionally read as 0.00 even when Object Count for the bucket is greater than 1.
The field itself is pulled from Usage data in the Cost and Usage Report (CUR) across the different storage types e.g. Glacier Storage and Standard Storage, but is rounded down to 2 decimal places. This means any buckets with objects present but the storage value is below 10.24 Mega Bytes (0.01GB) will show 0.00 for that bucket.
This is expected behavior for the platform. Usage values for the bucket can be verified via the S3 Usage Report and using the most recent day.