I ran 15-20 app updates earlier today and they all went through (iPhone 6 on 8.4 JB w/ TaiG 2.20) I haven't tried to download any new apps yet but my updates did work.
Did you both use TaiG or Pangu?
Did you both install the '3k' option in Taig if thats the source you used? (Not sure if relevant, but I know its an alternate app store so it could interfere perhaps)
Make sure you're logged into App store. No idea why it doesn't prompt you to sign in when installing new apps since all apps restored fine after restoring 8.4 as new.
I appreciate the replies as well. I suspect my jb has other issues going on. It wont let me connect to the app store when I try to log on and sometimes my keyboard is in spanish.
Just JB my iPad and I was faced with the same problem. It appears the JB signs you out of the App store so I just signed in again in Settings. It worked after a few attempts to update.
Sorry guys if its off topic. i just jb my iPhone 6 on iOS 8.4 and noticed that Cydia Substrate is missing at installed section. Now I'm scared to load my tweaks back. Pls let me know.