Just to be certain, have your checked your messaging settings and ensured group text is still on? Also you will need to have MMS automatically download for group chats.
I too am having this issue on my Lumia 1520 on developer preview. I can send mms to an individual, but not group messages as mms. Refreshing this option does not seem to fix it.
On the last OS version, I was receiving responses from myself. Perhaps its a related issue.
If you were to go to your Cellular+Sim settings, did your Sim settings change as well as your Internet APN/MMS APN? If so or not sure, you should contact your service provider.