Not receiving cookie that was previously set - node.js

Question

I have a very mysterious puzzle here. I monitor user activity, and record their actions to user profiles, simple. I use a cookie, as the user's id. If I've never seen the user before, I issue a random value as a cookie. After that, if/when I see that guy again, I rewrite their cookie value on all subsequent visits. Pretty standard stuff so far. Mysteriously, after a few requests, very rarely, the inbound cookie is just not there as a part of the request. Here is a truncated view of my access logs for a given user:
2015-02-19 12:14:06 ip:108.45.172.178 cookie:tx7Xd8e8InQNyceDFPgWHS1424376343 /path1 "othercookie=nk1bwo:fxx-2gop9,nk1btk:fxx-2gop8; userid=tx7Xd8e8InQNyceDFPgWHS1424376343; HASESSION=8888; AWSELB=<LONGSTRING>"
2015-02-19 12:14:07 ip:108.45.172.178 cookie:tx7Xd8e8InQNyceDFPgWHS1424376343 /path2 "othercookie=nk1bwo:fxx-2gop9,nk1btk:fxx-2gop8; userid=tx7Xd8e8InQNyceDFPgWHS1424376343; HASESSION=8888; AWSELB=<LONGSTRING>"
2015-02-19 12:14:07 ip:108.45.172.178 cookie:tx7Xd8e8InQNyceDFPgWHS1424376343 /path3 "othercookie=nk1bwo:fxx-2gop9,nk1btk:fxx-2gop8; userid=tx7Xd8e8InQNyceDFPgWHS1424376343; HASESSION=8888; AWSELB=<LONGSTRING>"
2015-02-19 12:14:08 ip:108.45.172.178 cookie:ybJRVsoDVxzJJ6SsdaaZSS1424376845 /path4 "HASESSION=8888; AWSELB=<LONGSTRING>"
2015-02-19 12:14:09 ip:108.45.172.178 cookie:ybJRVsoDVxzJJ6SsdaaZSS1424376845 /path5 "HASESSION=8888; AWSELB=<LONGSTRING>; userid=ybJRVsoDVxzJJ6SsdaaZSS1424376845"
2015-02-19 12:14:10 ip:108.45.172.178 cookie:ybJRVsoDVxzJJ6SsdaaZSS1424376845 /path6 "HASESSION=8888; AWSELB=<LONGSTRING>; userid=ybJRVsoDVxzJJ6SsdaaZSS1424376845"
2015-02-19 12:14:11 ip:108.45.172.178 cookie:ybJRVsoDVxzJJ6SsdaaZSS1424376845 /path7 "HASESSION=8888; AWSELB=<LONGSTRING>; userid=ybJRVsoDVxzJJ6SsdaaZSS1424376845"
2015-02-19 12:14:12 ip:108.45.172.178 cookie:ybJRVsoDVxzJJ6SsdaaZSS1424376845 /path8 "HASESSION=8888; AWSELB=<LONGSTRING>; userid=ybJRVsoDVxzJJ6SsdaaZSS1424376845"
In order, you can see a timestamp, then ip, then the cookie value I'm using for that request, then the obfuscated path (lots of ugly irrelevant GET params), and then finally a dump of the inbound cookies on the request.
On the first request, this user has a userid cookie set, along with another irrelevant cookie, a cookie set by HAProxy for sticky sessions, and a cookie set by AWS ELB (138 characters long).
Then go to #4, I still have the haproxy cookie, and the awselb cookie, but the userid and the othercookie aren't part of that request? Where did they go? Consequently, I generate a new userid value, write it as part of the response, and that is subsequently received for all further requests. What's going on?
Here's another log showing what I'm writing to the header on outgoing responses for a different guy. I'm logging this immediately before my res.setHeader('Set-Cookie', cookie_list) line:
2015-02-18 17:47:47 ip:66.56.52.46 cookiessetting: ["userid=HxfmJNtK7rSXUB1vOnGDkC1424310323; Domain=.mydomain.net; Path=/; Expires=Sat, 18 Feb 2017 01:47:47 GMT"]
2015-02-18 17:47:48 ip:66.56.52.46 cookiessetting: ["userid=HxfmJNtK7rSXUB1vOnGDkC1424310323; Domain=.mydomain.net; Path=/; Expires=Sat, 18 Feb 2017 01:47:48 GMT"]
2015-02-18 17:47:49 ip:66.56.52.46 cookiessetting: ["userid=HxfmJNtK7rSXUB1vOnGDkC1424310323; Domain=.mydomain.net; Path=/; Expires=Sat, 18 Feb 2017 01:47:49 GMT"]
2015-02-18 17:47:50 ip:66.56.52.46 cookiessetting: ["userid=HxfmJNtK7rSXUB1vOnGDkC1424310323; Domain=.mydomain.net; Path=/; Expires=Sat, 18 Feb 2017 01:47:50 GMT"]
2015-02-18 17:47:51 ip:66.56.52.46 cookiessetting: ["userid=X0AF14Lbz6sJ8nh45C7Tws1424310463; Domain=.mydomain.net; Path=/; Expires=Sat, 18 Feb 2017 01:47:51 GMT"]
2015-02-18 17:47:53 ip:66.56.52.46 cookiessetting: ["userid=X0AF14Lbz6sJ8nh45C7Tws1424310463; Domain=.mydomain.net; Path=/; Expires=Sat, 18 Feb 2017 01:47:53 GMT"]
2015-02-18 17:47:54 ip:66.56.52.46 cookiessetting: ["userid=X0AF14Lbz6sJ8nh45C7Tws1424310463; Domain=.mydomain.net; Path=/; Expires=Sat, 18 Feb 2017 01:47:54 GMT"]
2015-02-18 17:47:56 ip:66.56.52.46 cookiessetting: ["userid=X0AF14Lbz6sJ8nh45C7Tws1424310463; Domain=.mydomain.net; Path=/; Expires=Sat, 18 Feb 2017 01:47:56 GMT"]
This shows the same thing, the request at 17:47:51 came in without that inbound cookie, so it generated a new one and that got used for all subsequent requests. How does the cookie setting look? I'm pretty sure that how it works ....
My set up is that I have AWS ELB directing requests to a fleet of ec2 instances, on each ec2 instance, I have HAProxy as a reverse proxy going to a bunch of different open ports. The ports/processes are node.js/express. Pretty standard technologies.
Do cookies like this sometimes just get lost? Do other players on the internet (isp's) do bad things like this? Why would they drop my userid cookie and not the AWSELB one? I'm really stumped on this one, and I would really appreciate some help here ....
EDIT
This is my code to do the cookie setting, I know there are newer ways to do it in express, but this should work right?:
var futuredate = new Date().getTime() + 2*365*24*60*60*1000;
var datestring = new Date(futuredate).toUTCString()
var cookie_list = []
cookie_list.push('userid ='+cookieval+'; Domain=.mydomain.net; Path=/; Expires='+datestring);
if(someconditional)
cookie_list.push('othercookie ='+val+'; Domain=.mydomain.net; Path=/; Expires='+datestring);
res.setHeader('Set-Cookie', cookie_list)