iRule to insert header ntCoent-Length based in URI

I spotted several issues in your iRule. First, HTTP_RESPONSE occurs before HTTP_RESPONSE_DATA. (See https://devcentral.f5.com/s/question/0D51T00006i7X94/irule-event-order-http for more info.) That means, the HTTP::header insert for ntCoent needs to occur in the HTTP_RESPONSE_DATA event rather than in the HTTP_RESPONSE event. Second, HTTP_RESPONSE_DATA occurs after an HTTP::collect command. You have one HTTP::collect command but it is in the HTTP_REQUEST event, so it applies to the client-side request payload, not the server-side response payload.

I tested an alternative scenario below that might work for you or at least get you started, since I don't have the benefit of testing with your application. I decided to log the Content-Length header as sent by the server, too. I could not get the iRule to trigger the HTTP_RESPONSE_DATA event unless I specified a value on the HTTP::collect command:

when HTTP_REQUEST {
    # Set default pool and no URI match
    set urimatch 0
    pool pool_default
    if { [class match [HTTP::uri] starts_with "datatag_A"] } {
        pool pool_A
    } elseif { [class match [HTTP::uri] starts_with "datatag_B"] } {
        pool pool_B
    } elseif { [class match [HTTP::uri] starts_with "datatag_C"] } {
        pool pool_C
        # This request needs an ntCoent header inserted in the response
        set urimatch 1
    }
}
 
when HTTP_RESPONSE {
    if { $urimatch } {
        log local0. "HTTP RESPONSE Content-Length header is [HTTP::header Content-Length]"
        # Adjust as needed. Will not work with no value
        HTTP::collect 100000
    }
}
 
when HTTP_RESPONSE_DATA {
    if { $urimatch } {
        log local0. "HTTP RESPONSE DATA payload length is [HTTP::payload length]"
        HTTP::header insert ntCoent [HTTP::payload length]
        HTTP::release
    }
}

...and here is the log output it produced when just 1 of the URI's I requested matched my Data Group C:

Jun 21 10:48:49 bigip4 info tmm[9908]: Rule /Common/test_devcentral_irule <HTTP_RESPONSE>: HTTP RESPONSE Content-Length header is 22666
Jun 21 10:48:49 bigip4 info tmm[9908]: Rule /Common/test_devcentral_irule <HTTP_RESPONSE_DATA>: HTTP RESPONSE DATA payload length is 22666

 So, at that point, you might as well just pass the value contained in the Content-Length header. If I used a lower value, such as HTTP::collect 1, the HTTP_RESPONSE_DATA event still only triggered once, and there was a discrepancy between the Content-Length header and the HTTP::payload length, as expected. I tested on BIG-IP v14.1.0.