MSTP Topology

We have 4 HP switches, 2 ProCurve (SW1, SW2) and 2 ArubaOS-CX (SW3, SW4), connected in a ring and MSTP is not behaving how we expected it to.

All four switches have two instances configured, MST0 and MST1. The two ProCurves are in a region and the two CXs are in individual regions.

For MST0, SW1 is regional root, as expected, with priority of 4096. SW2 is priority 8192. SW3 and SW4 are priority 28672.

For MST1, SW1 and SW2 are priority 16384, SW03 is priority 4096, SW4 priority is 8192.

MST0 and MST1 have the same topology though.

SW1 -> SW2 Forwarding, SW1 -> SW3 Forwarding, SW1 -> SW4 Forwarding

SW2 -> SW1 Forwarding, SW2 -> SW3 Forwarding, SW2 -> SW4 Forwarding

SW3 -> SW1 Root Forwarding, SW3 -> SW2 Alt Blocking, SW3 -> SW4 D Forwarding

SW4 -> SW1 Root Forwarding, SW4 -> SW2 Alt Blocking, SW4 -> SW3 Alt Blocking

With the priorities configured, we expected SW1 to be root for MST0 and regional root for the region containing SW1 and SW2. We expected SW3 to be root for MST1 and SW4 -> SW3 to be Root Forwarding because they are directly connected.

Does anyone have any ideas?