Ok, so I closed all of my web browsers just after I posted last and just now reopened them (to a default homepage) starting with internet explorer this time. None of them gave the error so I rebooted and tried again. Still didn't recreate the issue. After clearing caches and rebooting it would seem not to be a client side or related issue but I'll try again tomorrow just to be sure it isn't chrome.

I've also gone through my logs for May and April and looked at each status code that appears. It doesn't look like any of them are related to the issue.

I found 200,206, and 304 codes that all appear to be normal working statuses.
I've found 404s but confirmed using regex and match counts that all instances of them are for the nonexistant favicon.ico.
There was also a single 406 code generated by a request from a discord bot rather than from a person.

The error code ERR_NAME_NOT_RESOLVED leads me to believe that request didn't make it to the site the first time. Maybe it has something to do with the way subdomains are resolved into ip addresses?

If you still think it may be php related and if you don't already have access to them, I could send you the logs. Just let me know how to get them there.

Thanks again for your help so far.

Thanks again for your help. I did setup the subdomain last Sunday (4/29/2018) and last recreated the problem on Thursday (5/3/2018) but I wasn't able to recreate it today.

Hopefully thats all it was and its worked its self out but I'll check on it over the next couple of days and contact support if it happens again. If it does, I'll also post whatever I find here incase its of use to anyone else in the future.