Request Logging Profile - Respond On Error

hi there, I can't seem to get the Respond On Error feature to work. The template works fine. When I do a tcpdump I don't see the error response being modified. I also don't see any errors being logged either. I m on v12.1 LTM VE. Any ideas?

ltm profile request-log my_request_log {
    app-service none
    defaults-from request-log
    log-request-logging-errors yes
    log-response-by-default yes
    log-response-logging-errors enabled
    proxy-close-on-error no
    proxy-respond-on-logging-error yes
    proxy-response "  ERROR 
 503 ERROR-Service Unavailable 
"
    request-log-error-pool request_pool
    request-log-error-protocol mds-udp
    request-log-error-template none
    request-log-pool request_pool
    request-log-protocol mds-udp
    request-log-template "Client for request settings $CLIENT_IP http request $HTTP_REQUEST  ,http status code $HTTP_STATCODE, http status $HTTP_STATUS"
    request-logging enabled
    response-log-error-pool request_pool
    response-log-error-protocol mds-udp
    response-log-error-template none
    response-log-pool request_pool
    response-log-protocol mds-udp
    response-log-template "Client for response settings $CLIENT_IP http request $HTTP_REQUEST  ,http status code $HTTP_STATCODE, http status $HTTP_STATUS"
    response-logging enabled
}

tcpdump result. Disregard the checksum error as it occurs even under normal conditions.

10.10.1.100.80 > 10.10.1.130.35445: Flags [P.], cksum 0x1808 (incorrect -> 0                                                                                                                     xe545), seq 1:233, ack 282, win 4661, options [nop,nop,TS val 2229884405 ecr 145                                                                                                                     9866], length 232 out slot1/tmm1 lis=/Common/http_vs
    0x0000:  4500 011c 2b47 4000 ff06 389b 0a0a 0164  E...+G@...8....d
    0x0010:  0a0a 0182 0050 8a75 b5d0 1a65 4bc1 ee9e  .....P.u...eK...
    0x0020:  8018 1235 1808 0000 0101 080a 84e9 55f5  ...5..........U.
    0x0030:  0016 469a 4854 5450 2f31 2e30 2035 3030  ..F.HTTP/1.0.500
    0x0040:  2049 6e74 6572 6e61 6c20 5365 7276 6572  .Internal.Server
    0x0050:  2045 7272 6f72 0d0a 4461 7465 3a20 5468  .Error..Date:.Th
    0x0060:  752c 2032 3720 4a75 6c20 3230 3137 2031  u,.27.Jul.2017.1
    0x0070:  363a 3338 3a31 3220 474d 540d 0a53 6572  6:38:12.GMT..Ser
    0x0080:  7665 723a 2041 7061 6368 652f 322e 342e  ver:.Apache/2.4.
    0x0090:  3130 2028 4465 6269 616e 2920 4f70 656e  10.(Debian).Open
    0x00a0:  5353 4c2f 312e 302e 3174 206d 6f64 5f70  SSL/1.0.1t.mod_p
    0x00b0:  6572 6c2f 322e 302e 3964 6576 2050 6572  erl/2.0.9dev.Per
    0x00c0:  6c2f 7635 2e32 302e 320d 0a43 6f6e 7465  l/v5.20.2..Conte
    0x00d0:  6e74 2d4c 656e 6774 683a 2031 0d0a 436f  nt-Length:.1..Co
    0x00e0:  6e6e 6563 7469 6f6e 3a20 636c 6f73 650d  nnection:.close.
    0x00f0:  0a43 6f6e 7465 6e74 2d54 7970 653a 2074  .Content-Type:.t
    0x0100:  6578 742f 6874 6d6c 3b20 6368 6172 7365  ext/html;.charse
    0x0110:  743d 5554 462d 380d 0a0d 0a0a 0114 0100  t=UTF-8.........
    0x0120:  0001 0f2f 436f 6d6d 6f6e 2f68 7474 705f  .../Common/http_
    0x0130:  7673                                     vs