Spaning Tree MST, why are 2 switches root in the same instance?

Question

Cisco

The network has 2 switches Core (Catalyst 3750) and Aktsess (Catalyst 2950)
. Three MSTs are configured in them. The fig is that the core is the root of all instances, but it turns out that the access also considers itself the root for the first instance.
How is it that in the same instance 2 switches choose themselves as the root?
Cat 3750
MST0
Spanning tree enabled protocol mstp
Root ID Priority 28672
Address a493.4c7f.7980
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 28672 (priority 28672 sys-id-ext 0)
Address a493.4c7f.7980
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
MST1
Spanning tree enabled protocol mstp
Root ID Priority 4097
Address a493.4c7f.7980
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 4097 ( priority 4096 sys-id-ext 1)
Address a493.4c7f.7980
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
MST2
Spanning tree enabled protocol mstp
Root ID Priority 24578
Address a493.4c7f.7980
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 24578 (priority 24576 sys-id-ext 2)
Address a493.4c7f.7980
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Cat 2950
MST00
Spanning tree enabled protocol mstp
Root ID Priority 28672
Address a493.4c7f.7980
Cost 0
Port 65 (Port-channel1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768 (priority 32768 sys-id-ext 0)
Address 0015.2bf9 .e940
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
MST01
Spanning tree enabled protocol mstp
Root ID Priority 32769
Address 0015.2bf9.e940
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0015.2bf9.e940
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
MST02
Spanning tree protocol enabled mstp
Root ID Priority 24578
Address a493.4c7f.7980
Cost 10000
Port 65 (Port-channel1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32770 (priority 32768 sys-id-ext 2)
Address 0015.2bf9.e940
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
LAN_CORE_3750X#sh run
Building configuration..
spanning-tree mode mst
spanning-tree extend system-id
!
spanning-tree mst configuration
name OPFR
revision 1
instance 1 vlan 2-199
instance 2 vlan 200-300
!
spanning tree mst 0 priority 28672
spanning tree mst 1 priority 4096
spanning tree mst 2 priority 24576
spanning-tree vlan 1-9,11-199 priority 4096
spanning-tree vlan 200-300 priority 24576
sw_2950_MNGX#sh run
Building configuration...
spanning-tree mode mst
no spanning-tree optimize bpdu transmission
spanning-tree extend system-id
spanning-tree vlan 2-4094 priority 61440
!
spanning-tree mst configuration
name OPFR
revision 1
instance 1 vlan 2-199
instance 2 vlan 200-300

Answer 1

Do you have 15 IOS on both switches?
There, some IOS considered hash according to the prestandard like.
Indeed, in order for mst to converge, it is necessary that the hash match, and on different versions it can be calculated differently.
If the switches are both root, then they have created independent mstp domains, that is, in fact, different instances.

Answer 2

Try like this, without specifying priority:

spanning-tree mst instance-id root {primary | secondary} [diameter dia [hello-time hello-time]]