The new bone formed from anabolic meds is still suseptible to bone loss due to your underlying reason for bone loss to start with. Which could be loss of estrogen. So even with new bone formed your rate of bone loss can still be high. Which is why they recommended taking bisphosphonate to lock in gains. As this slows the bone resorption (breakdown) process.
Slowing down the process should keep your bones stronger longer.
That is my understanding.
Since no cure for osteoporosis, they should continue to monitor after all treatments complete and may recommend additional meds in the future.