An Amazon EC2 instance has been configured with an IAM instance role that grants it both read and write permissions to the Amazon S3 bucket named 'my_bucket'. Upon testing, it is confirmed that the IAM instance role successfully allows both reading from and writing to the bucket. Subsequently, the IAM role is detached from the EC2 instance, and another test is conducted to check the read and write functionalities. It is observed that write operations fail while read operations continue to work.

What is the most probable explanation for this observed behavior?