We are load balancing the DNS Servers on the NetScaler and when we update a record on the Primary DNS server and then the SOA record, the Slave DNS Servers are not getting updated with the latest serial no.. It appears like the Slave DNS Servers are getting cached response from the NetScaler

Since we don't want the NetScaler to perform any DNS specific features, we removed the Load Balancing virtual server and the services configured using DNS Protocol. 
Next, the Load Balancing virtual server and the services were added using UDP protocol so that the NetScaler performs Layer 4 Load balancing only.

Also, since the Slave DNS Server were aware about the Primary DNS server as the propagation host, we had to enable USIP on the load balanced services. This is an optional step and can be kept disabled if the Slave servers are made aware about the Source IP for the propagation to be the SNIP.

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Problem Cause

The configuration was reviewed where it was found that the DNS Servers are load balanced using DNS protocol. Since the DNS Servers are load balanced using the DNS protocol, the NetScaler would validate packet format of each DNS request and response. Also, the NetScaler would cache DNS response as well.

When we saw the SOA records on the NetScaler, we could see the NetScaler with the outdated SOA record that was causing the issue.