Externip not working in Asterisk 10.3.1

When using NAT, I’m setting my local subnet and externip using the following:

[general]
bindport=5060
bindaddr=10.27.1.213
externip=68.188.108.40
localnet=10.27.0.0/255.255.248.0
media_address=208.85.62.102
transport=udp

OPTIONS pings are still using my private addrress (10.27.1.213) instead of the externip (68.188.108.40):

<— SIP read from UDP:208.85.62.102:5060 —>
OPTIONS sip:metaswitch@10.27.1.213:5060;transport=udp SIP/2.0
Via: SIP/2.0/UDP 208.85.62.102:5060;branch=z9hG4bK8gefav0078vgfks2b2q1.1
Allow-Events: message-summary, refer, dialog, line-seize, presence, call-info, as-feature-event
Max-Forwards: 69
Call-ID: 676B89C0@v.cfly.co
From: sip:metaswitch@peer.cfly.co:5060;tag=v.cfly.co+1+1ffc01+bd052356
CSeq: 810575716 OPTIONS
Organization: Metaswitch Networks
Supported: resource-priority, 100rel
Content-Length: 0
Contact: sip:metaswitch@208.85.62.102:5060;transport=udp
To: sip:metaswitch@10.27.1.213

<------------->
— (12 headers 0 lines) —
Looking for metaswitch in default (domain 10.27.1.213)

<— Transmitting (NAT) to 208.85.62.102:5060 —>
SIP/2.0 200 OK
Via: SIP/2.0/UDP 208.85.62.102:5060;branch=z9hG4bK8gefav0078vgfks2b2q1.1;received=208.85.62.102;rport=5060
From: sip:metaswitch@peer.cfly.co:5060;tag=v.cfly.co+1+1ffc01+bd052356
To: sip:metaswitch@10.27.1.213;tag=as759f2c7a
Call-ID: 676B89C0@v.cfly.co
CSeq: 810575716 OPTIONS
Server: Asterisk PBX 10.3.1
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY, INFO, PUBLISH
Supported: replaces, timer
Contact: sip:68.188.108.40:5060
Accept: application/sdp
Content-Length: 0

I have this exact same setup working at another site that is using Asterisk 1.6.1.

Has something changed or broke with Asterisk 10.3.1 where externip no longer works?