Another data point: Within powershell, curl (alias for Invoke-WebRequest) is able to successfully download a failed link from the log and save it to a file, but there is a significant delay (2-3 minutes) before it returns to the prompt and the file is there.
I also found a few links for other software (MythTV and Plex) having similar issues but I didn’t see anything that I could try. The Sep30 date for the phase-out of the old TheTVDB API, does roughly coincide with when I noticed new shows missing artwork.
https://lists.gt.net/mythtv/users/613912
https://www.mythtv.org/wiki/TheTVDB_API_v2
https://forums.thetvdb.com/viewtopic.php?f=17&t=42599
I should also say that there are a very small number of posters slipping through and getting downloaded. Since I cleared out the MediaCover folder a couple of weeks ago, one out of every 5-10 shows now has a poster displayed in the series tab of the GUI. I want to say that half of them popped up soon after clearing the folder, with the rest coming at no discernable interval (not 1/day, one was a new series add but other new series fail). There is also no pattern in the shows with posters displayed (old vs. new, series age, date added to sonar, alphabetical).