A new access switch ignores its MST instance and sends VLAN 20 the long way

Last month the campus moved from Rapid PVST+ to MST: region CAMPUS, revision 3, instance 1 for VLANs 10 to 19 rooted on DSW1, instance 2 for VLANs 20 to 29 rooted on DSW2, and DSW1 also root for instance 0. HSRP active follows each instance's root. Today a new engineer added ASW3 for the east wing. Everything works, but VLAN 20 users on ASW3 reach their gateway on DSW2 through DSW1, and ASW3's uplink to DSW2 carries no traffic. On ASW1, built to the same standard, the DSW2 uplink forwards for VLANs 20 to 29.