Play Store downloads failing for subset of apps
I have found a partial answer to this issue. It appears to be related to a google play error reported as error 963 elsewhere online. I'm not sure how you get the associated error number as I get no message or sign that anything is wrong other than the download being attempted three times and stopping on the third try?

After looking through endless sites suggesting the same old solution of clearing cache and data and uninstalling updates I came across a site that mentions the issue is related to the app being stored on the SD card.

https://www.technipages.com/google-play-error-963

Following these instructions worked for me. Basically, all the apps that were not updating were stored on my SD card. After moving them all back to internal storage I could once again successfully update them. 

The only remaining issue I have is if I try and move them back to the SD card (either via the app storage preference or via the auto "you're running out of internal storage move apps to SD card" thing that pops up from time to time) then the next time the app gets an update it fails to complete again and I have to move it back to internal storage to get it to update.

I can only surmise that for some reason when the app is on the SD card google play doesn't have the required permissions to update the data. Having said that it is strange that uninstalling updates on google play also fixes the issue as that suggests that the factory version has the required permissions and this bug has been introduced at a later date.