Humm,
As far as I am aware no telephone carrier service, in any country, has ever used a form of Notification that a Text Message (SMS) has even been delivered.
The Messages app on the iPhone (and those iPads with SIM Cards) can tell if the item cannot go as an iMessages.
There is a connection to the server, similar to Buddy Lists in other Services like Jabber or AIM or Yahoo, where the app gets feedback about who is on line.
Anyone who is Off Line for whatever reason generates the "Not Registered at iMessages" alert.
However if you have SMS on then the item will be sent to any number, as whether it exists or not is never checked.
The app itself tries it's best to deliver whatever you typed.
If you don't know what your Contact needs in terms of the delivery method it is hardly the App's fault.
I find it difficult to see that as a Bug.
If the Mac version is not connecting to the iPhone for any SMS item then you will need to check and possible reset the Link between the iPhone and the Mac.
This might be different Apple IDs on each device,
The iPhone not being on the same WiFi network as your Mac,
Continuity or Handoff not being set.
Or Text Forwarding not even being set.
If it is only to this one Contact then it can be very difficult to pin down particularly if you don't even know if they have an iPhone or even SMS service.
Keep us up to date on the Bug report if you get any feedback.

8:27 pm Saturday; March 3, 2018
iMac 2.5Ghz i5 2011 (Sierra)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone 6 iOS 11.x and an iPad (2)