Offline OMEMO message decryption

Hello, currently i am using Smack 4.2 with Openfire 4.3.2 and the Monitoring plugin 1.7.0 (XEP-0313) to store OMEMO encrypted group chat messages for later retrieval. The problem that i am facing is that when a client is offline he is not considered a part of the chat room in accordance to XEP-0045, so when the message is encrypted his key is not present in the header tag. As a result when the user receives the message he is not able to decrypt it. It this expected behaviour or am i missing something?
Thank you!