tekeek

I had forgotten all about the sleeping tabs. I went into Edge Settings > System and performance adding [*.]testmy.net to the "Never put these sites to sleep" and it has successfully made it farther than before.

I wonder if there are new power management settings and Edge may be putting the tab to sleep or something.  I'll put it on the bench and see if I can reproduce and get back with you.
 
For now, try scheduling the same test using Chrome or Firefox and see if this resolves the issue.  
 
If there are new settings I may be able to work around them.
 
Sorry for the extremely delayed response.  I got your message right away and 10 minutes after you sent it I started scheduling some tests and investigating but wasn't able to reproduce the issue (but wasn't using Edge).  I had it scheduled out for a while and then by the time it finished forgot to update this thread.  My bad.  -- Again, I'll give it a try using Edge, may provide more info.  Pretty certain it's not an issue effecting large numbers of users.