I received a prompt, non-automated, and informative reply from TP-Link support. Actual support is so rare these days that I must give kudos for that.

Unfortunately the answer is this:

"We did the test. The log of ER605 is not recorded according to the results of online detection, but is determined according to the connection status in System Status. When the WAN port status is link down, it will be recorded in log. Every time the network cable directly connected to the ER605WAN port is plugged in or unplugged, the log will show the record. After unplugging the network cable, please check the corresponding WAN port status in the system status. Is it link down? If so, check if there is any record in the log."

So apparently this is a feature, not a bug.

I find this absurd. Perhaps it is different when using Omada (something I do not wish to do), but when in stand alone mode this unit provides no way of informing the user that its primary function, namely multi WAN support, has failed for any reason other than the cord physically being removed. Any other type of failure is just not worth mentioning or being informed of I guess. No email, no pop-up, no mention in the log. Nothing at all. By design.

I'll leave this info here for the next person who goes looking for it, but as for me I'll be moving along. Shame, since I really wanted to like this unit, but I need to know when my WAN is down.
 

@CutterKin Just one more notch on the "Sub par logging by TP-Link hardware", you are not alone in finding this a deal breaker when considering TP-Link hard/Software for Enterprise settings. And just to asure you, it's not a "standalone" issue, happens also when adopted to a controller. 

Thank you for reporting. I was looking for information about this. 

It would be great to see this addressed in an update. While consistently logging this item may not be critical to functionality, not having it in place is blocking adoption of your hardware by some thoughtful users.  

I find it a deal breaker too. Without proper logging of primary WAN failure it is not possible to raise issue with ISP about how frequently this is happening.

My primary ISP, I am guessing, fails about once in two hours for a minute or so. If ER605 logs these failures properly i will be able cite the log and complain with proper time stamps. 

Wow, what a big thing to leave out. Count me as another user that will be returning this thing to Amazon. I really thank you all for posting about it here because I just spent the last few hours trying to figure out what I was doing wrong, but it truns out the problem is in the router.

How did this get past testing and quality control? I think you all are being way too polite about it. This is not a small bug to me - it is  a flagrent defect and I can't believe that TP-Link is just ignoring it since it looks like it was reported almost a yeat ago. Completely horrible design decision and I guess at least I found it out early on so I don't waste anymore time with this junk. TP-Link you should fix this dumb issue.