Why is iMessage splitting group conversations into multiple threads after upgrading to iOS 8?

After upgrading to iOS 8 my group messages keep getting split up into different threads. Everybody in the threads have iPhones and before upgrading they were all working and sending/receiving iMessages just fine.

Now I'll have 2-3 threads with the same contacts (using their same phone numbers, not another contact point like iCloud email, Apple ID, etc.). My phone sometimes recognizes that the group is the same because it will automatically name the group the same thing, but other times it just puts the names of the people in the "To" line. In addition, some of the threads won't let me send iMessage so I have to reply in one thread and receive messages in the other (I send iMessages in thread A but the replies from the same people come back in thread B, but if I try and reply in thread B I can only send text messages even though all contact info is the same).

Update after OS X Yosemite: After upgrading to Yosemite, my Messages app on my MacBook Pro is now splitting the threads as well. It used to be that the computer understood the contacts were the same so all messages should be under one thread and just my phone was having this issue, but the upgrade changed that.

I have tried signing in and out of iMessage, upgrading to the latest iOS, turning on and off my phone, starting new groups, only replying to certain groups, etc. but nothing has worked.

Thanks!