This behavior is by design. This excerpt from "SQL Server Replication: Providing High Availability using Database Mirroring" written by Paul Randal and Gopal Ashok, explains it well

"It is possible for the principal to become exposed (the mirror is accessible but there are log records that have not yet been hardened on the mirror) or isolated (the mirror is inaccessible). In both cases, if the principal is still able to serve the database, any changes made to the database are not replicated until the corresponding log records are hardened on the mirror.

This behavior introduces latency in the replication stream, and is done so that if a mirroring failover occurs, it is guaranteed that replication Subscribers cannot be “ahead” of the new principal database."