Internal server to VIP from other than Internal subnet

I agree with nitass.

If you do not want to configure SNAT Automap full time then you can use an iRule to engage SNAT when it is talking to another device on a different subnet of the same device.

Create a Data Group (mynetwork in this example) and add the subnets for the BigIP into it. This example is for v10.x.x.

when CLIENT_ACCEPTED {
if { [class match [IP::client_addr] equals mynetwork] } {
snat automap
}
}