@Venkatesha Bhat , There looks like a bug in the portal. I have tried this using CLI and was able to add multiple IPV6 addresses to the front end config of standard load balancer.
Here is the script for quick reference,
# Create IPv6 front-end
az network lb frontend-ip create \
--lb-name dsLB \
--name dsLbFrontEnd_v6 \
--resource-group DsResourceGroup01 \
--public-ip-address dsPublicIP_v6
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