Very common.

Also I already had 15.11 installed, and got a prompt for new update on the app, it was an update for the already 15.11, was it a new build version, or is the app just screwed up?

There was a firmware only update of 15.11 without an app update a few days ago. Just the build nummer changed. 
Could be you’re talking about that instead of an app update? App update would have been shown in Apple appstore. The firmware update was shown within Sonos app. 

@ledzep1 

Regarding the „failed update“ message… don’t worry! I talked to Sonos about this and they didn’t change the time period the app waits for response of all devices after the update for a long time. But with the latest updates the bonded home theater products (sub and rearspeaker) now take more time to reconnect than in the past. So for the app there’s a timeout showing this error message. But the bonded ht devices get the update well and just need some more time to reconnect to the ht master device after an update. 
I hope Sonos will fix this with an app update to not confuse users. 

Yes. This. OP is talking about updating within the Sonos app. This happens to me every single time there is an update. Makes me think something is actually wrong with my network or Sonos system. Just strange that it says error, but try again “no update available”. Really hope they would address this issue. 

@Obviousfrog 

I think now they are aware of it and hopefully addressing it with a further update. 
To get sure they know about you can call support team and point to this thread. Maybe also @Corry P or another Sonos mod can push it to the devs again.