Well, you are kindly advised to check the dns resolve configuration and routes of the dns server.
VPN-instance doesn’t support encapsulate the DNS packet, so there should be a route to direct the DNS packets from the public instance to the vpn-instance. Please refer the configuration below:
update host source ip x.x.x.x vpn-instance PUB_ISP_vod
dns server vpn-instance PUB_ISP_vod
dns resolve
dns server 1.1.1.1
ip route-static 1.1.1.1 32 vpn-instance PUB_ISP_vod x.x.x.x