In Software V2.1, if you had engaged Cruise Control with the “Set” button*, it seemed to ALWAYS require a subsequent press of the “Travel Assist” button to switch the ID.4 into its self-driving mode.
Now, with V3.1, if you engage Cruise Control with the “Set” button, then depending on whether you last had just Cruise Control or full Travel Assist engaged, it will immediately re-enter that mode. That is, if you were last cruising in full Travel Assist and you press the “Set” button, you will immediately return to self-driving mode.
*I don't know if the on/off setting of ACC (“Adaptive Cruise Control”) affects that choice; I never cruise in non-ACC mode.