Which device are you testing this on?
I tested with the hps_client_keypad example, added a printf("scan callback") within the callback and faced no issues with a 43438.
How do you make sure the reduced stack size in sdk-6.1 has no impact on user's existing application?
I believe you cannot guarantee that, that is why I suggest to allow user to set the stack size.
Especially, sometimes people needs bigger stack size in debug build.
Thanks for the feedback. I will forward your suggestions to the design team.