Reverse metamorphosis is a real thing.
Like most other hydrozoans, T. dohrnii begin their life as tiny,
free-swimming larvae known as planulae. As a planula settles down, it
gives rise to a colony of polyps that are attached to the sea-floor...
Jellyfish, also known as medusae, then bud off these polyps and
continue their life in a free-swimming form, eventually becoming
sexually mature...If the T. dohrnii jellyfish is exposed to
environmental stress, physical assault, or is sick or old, it can
revert to the polyp stage, forming a new polyp colony...
Theoretically, this process can go on indefinitely, effectively
rendering the jellyfish biologically immortal, although in practice
individuals can still die.
Your dragon / human life stages seem biologically unlikely and if I were reading your fiction I would be watching for some sort of metaphorical application to the real world - perhaps adults reverting to childhood roles in their mom's basement after life stressors.
Back to biology: in your world, dragons are the polyp (juvenile) analog and humans are the adult (jellyfish) analog. You ask if there are creatures that lose the ability to do metamorphosis. Reverse metamorphosis as your creatures (and Turritopsos) does is unusual and I could imagine a mutation could block the ability to revert. These creatures would be juveniles, then adults, then old adults, then die as most creatures do. If you wanted (and I dont know why you would) to stick to strict biology you could have a telomere-like cap on the number of cycles, or otherwise incur damage to the organs used to revert.
I assume in this scenario "humans" give birth to baby dragons. Perhaps a human pregnancy goes on longer than usual and instead of a baby dragon, a baby human is born. This is sort of like the mammal trick of having developmental stages inside the mother rather inside an egg as our ancestors did. The new type carries out the dragon developmental stage in utero. These humans cannot change back to dragons because they never really were dragons.