I would say start looking from this error propective. The VPN tunnel going down due to VPN monitoring.
Jun 5 07:18:19 SRX300-Remote_SITE kmd[10477]: KMD_VPN_DOWN_ALARM_USER: VPN VPN_POLICY from REMOTE_IP is down. Local-ip: LOCAL_IP, gateway name: GW_at_HQ, vpn name: VPN_POLICY, tunnel-id: 131073, local tunnel-if: st0.0, remote tunnel-ip: 192.168.254.250, Local IKE-ID: Local_IKE-ID, Remote IKE-ID: Remote_IKE-ID, Traffic-selector: , Traffic-selector local ID: ipv4_subnet(any:0,[0..7]=0.0.0.0/0), Traffic-selector remote ID: ipv4_subnet(any:0,[0..7]=0.0.0.0/0), SA Type: Static, Reason: VPN monitoring detected tunnel as down. Existing IPSec SAs cleared
As per your configuration the VPN monitor option is configure as optimized.
The VPN monitoring optimized option sends pings only when there is outgoing traffic and no incoming traffic through the VPN tunnel. If there is incoming traffic through the VPN tunnel, the security device considers the tunnel to be active and does not send pings to the peer. Configuring the optimized option can save resources on the security device because pings are only sent when peer liveliness needs to be determined. Sending pings can also activate costly backup links that would otherwise not be used.
Try removing Optimized option or removing VPN monitoring altogather.