Strange cache on DNS forward zone

I have a corporate DNS server using Bind.
I have 3 consul servers. Consul is listening on localhost port 8660 for DNS.
On these servers I have also DNSmasq to forward DNS request from public IP port 53 to Consul on localhost.

The configuration of Consul is very simple. I just activated the DNS port. When I check the DNS locally on port 8660 it is working fine.

The configuration of DNSmasq on each consul server is:

server=XX.XX.XX.XX (first ip of the corporate DNS)
server=XX.XX.XX.XX (second ip of the corporate DNS)
listen-address=XX.XX.XX.XX (ip of the server)

The configuration on the corporate DNS is:

zone "" {
    type forward;
    forward only;
    forwarders {
        XX.XX.XX.XX; (ip of the first consul)
        XX.XX.XX.XX; (ip of the second consul)
        XX.XX.XX.XX; (ip of the third consul)

When some entries are added or removed from consul, the DNS zone on the corporate DNS is updated quite immediately.

When all the entries for a service are deleted from consul, the corporate DNS removed all entries in his table (which is normal).
But if the entries are created again, the DNS from consul is updated but not the corporate DNS.
To update the corporate DNS, I should execute the command rndc flushtree