This is a direct result of the fact that M-Record information is not exchanged between the regions
By default, all VLANs are mapped to the IST. This represents the case of classic IEEE RSTP with all VLANs sharing the same spanning-tree. Other MSTP instances could be enabled, and they are referred to as Multiple Spanning Tree Instances (MSTIs). Every MSTI assign its own priorities to the switches and use its own link costs to come up with a private logical topology, separate from the IST. Since MSTP does not send MSTI's information in separate BPDUs, this information is piggybacked into the IST's BPDUs using special M-Record fields (one for every active MSTI). Using TLV’s (Type-Length-Value) those fields carry root priority, designated bridge priority, port priority and root path cost among others.