I asked for clarification as to why a conversation correctly started with my primary number would randomly change to sending from an other address or number. I was told "if the number becomes unavailable it will randomly choose another address or number to send from". Seems like avoiding the issue of admitting a bug. Anyway, they closed my case with the recommendation to remove all but my primary number, which doesn't really work for me. Not much else I can do at this point though.
Did they mean they only support a max of one phone number OR one email? My settings had my phone and five email addresses (every one I have on the phone plus an iCloud and a me.com one!).
If it's supposed to be 'phone' only, will that still work to an iPad, for example? Haven't reviewed how iMessage works on non-cell devices, my bad.