As you told in your question, the association of C is with B (not with A), so in the change member of C, make its association with B, and you will get your issue resolved.
It's because C has an association with B.
Let me know if you face any issues.
Hope it helps!
it works actually
I do everything exactly as you do and for reasons I don't understand, it doesn't work for me :/.
I have the same relationship defined in Domain Model, the same process in microflow - a new object (ExtraProfileData) is created but I don't know why it is not associated with ProfileSM :/ On the other hand the association between ProfileSM and User is created correctly.
Just to be sure, I even created the data copied from ProfileSM to ExtraProfileData after creating the first one and this data is indeed in the second one.
Oh well, I'll keep thinking about it, but I'm tearing my hair out alreadyOh well, I'll keep thinking about it, but I'm tearing my hair out already
In step 3, create C with an association to B an no step 4.
Go Make It