Interestingly I switched my language to English (United Kingdom) and I get the same results so the issue appears it may be related to the language setting.
Looking more and more like a bug then
I've not found a workaround with the "Alexa Says" action of Routines but I did find one with "Simon Says" (I was using this as part of an alarm clock app to make Alexa say things when the alarm went off) by enabling a skill called "Repeat After Me". But this has the disadvantage that you need to say "Alexa, ask Repeat After Me to say phrase" and obviously won't work with Routines.
Hmmm... that's not much good is it?
Before the 'Alexa says' addition, when I'd say 'good morning' it would run the routine and say 'ok' which is just as bad
Not sure what else we can do now?