Ok, got that, thanks. I don't think there is a bug, the reason being it appears to work normally for others, and it works consistently wrong for you, I feel the settings are not right, and as mentioned by someone earlier, its probably the issue of the right setup of reach at and start new from in settings.
Well, others are reporting it as a bug and I see it as such. Neither of us has ever experienced this before. I had the same settings on the same gear working perfectly, until iOS8. My husband changed to iPhone 6 but settings remained the same. I've had an iPad for years without experiencing this before so it has me stumped. Even a complete reset and setting it up as a new device (rather than restoring - just in case it was settings) didn't fix it.
Here is my setup
iPhone 5S, iPad Air Cellular (cellular off) rMBP
5S and iPad Air
Send as SMS is on
MMS is on
Send and Receive, You can be reached by iMessage at: in order and all ticked: cell number, an iCloud email address, Apple ID email address (xtra.co.nz)
Start new conversations from: cell number, other two unticked
iMessage address in preferences is my Apple ID email address, the right side settings are identical to the above "you can be reached at and Start new convo from
Hope that helps