When a VSX primary switch is powered off, it loses connectivity with management systems such as HPE Aruba Central. This causes the management system to trigger a "Switch Disconnected" alert because the switch is no longer reachable.
"Switch Reboot" alerts would indicate a restart, not power off.
"Switch Stack Conductor Change" refers to stack leadership changes, not VSX primary down.
"Switch STP Root Change" relates to Spanning Tree Protocol root bridge changes.
Hence, powering off the VSX primary causes a "Switch Disconnected" alert.
References:
HPE Aruba VSX Management and Monitoring Guide
Aruba Central Documentation on Alerts and Events
ArubaOS-CX VSX Best Practices

For a 1 Gbps link between two Aruba CX switches, the appropriate SFP port type is an SFP (Small Form- factor Pluggable). SFP+ and SFP28 are used for higher speeds such as 10 Gbps and 25 Gbps, respectively.
QSFP+ is for even higher speeds (40 Gbps).
SFP supports 1 Gbps (1000BASE-X)
SFP+ supports 10 Gbps
SFP28 supports 25 Gbps and above
QSFP+ supports 40 Gbps and above
Therefore, the correct port type for a 1 Gbps link is SFP.
References:
Aruba CX Transceiver and Port Type Compatibility Guide
HPE Aruba Networking Switch Hardware Documentation
Aruba Networking Best Practices for Link Speeds