Nope, not the case. I got the Do Premium Plus bundle and was being charged out of bundle for it (i.e. as two, separate items at full price)
Had to spend like an hour with Accounts as well as get my ticket escalated before it was resolved. The bundling is NOT automatic.
If it did not happen automatically then something went wrong that they had to fix. I was on a 4Mbps simple plus package. Upgraded my line to 10Mbps which means they removed the simple package and I was paying for 10Mbps and 20GB softcap separately then when I upgraded the cap to 50GB over the phone and the Do premium Plus discount was activated. When I upgraded my line speed to 20Mbps I saw the discount automatically changed to Do Elite.
So for me its been all automatic and hence if it does not happen automatically something went wrong.