BigIP cookie is blank

We are using a cookie based persistence to achieve sticky session on a virtual server with 2 nodes. It is working fine for most of the users. But, some users are complaining about getting frequent errors while browsing the application. While investigating the issue, we found out that the user's requests were going to both of the nodes in a single session and that was causing an error when the request went to the node which did not have session object for the user.

We tried to drill down further by capturing HTTP headers and cookies using a third-party APM and we observed that there were requests which had blank value for BigIP cookie while other cookies (set by application) seem to be fine. Is there any reason why this can happen?

Thanks, Rupendra