The CUSTS1_VAL_NTF_REQ message as documented in custs1_task.h:
/// Set/update characteristic value and trigger a notification
CUSTS1_VAL_NTF_REQ,
suggests that the handler for this message will actually set the value in the ATT database. This was the case with SDK 5 but appears to NOT be the case for SDK 6. In order to actually set the new value in the database, I need to set the value directly (using attmdb_att_set_value() or custs1_att_set_value()) before sending the notification message, or send a CUSTS1_VAL_SET_REQ to the custs1 task.
I would guess this is a bug. Is the SDK 6 intent to not set/update the characteristic value in the CUSTS1_VAL_NTF_REQ handler? If this is true, then the comment in the CUSTS1 message enum is misleading.
Device:

Hi mkelwood,
Yes, you are right on this, thanks for indicating, i will let the SDK team know.
Thanks MT_dialog