I have this problem with 8.0, just downloading 8.1 now. I will let you know if it persists!
8.1 is the one that I had problems with.
I checked XDA and a lot of people seem to be having issues....but there are a random few that it works perfectly for...hmm
I'm one that it works for. I did replace the mms.apk with one from Unnamed because I liked the theme better, so idk if that had anything to do with it. You can try it yourself if you like. I never got a clear answer to whether it helped anyone else over there.
You have to use something like Root Explorer to paste it into /system/app. After that, restart your phone and you should be good.
I'll try it again, maybe tonight. I did do this and all it did was it gave me the UnNamed MMS platform, but it didn't fix the MMS issue.
Do you have any issues reconnecting from being in airplane mode or from having data shut off?