What you describe is exactly what happens when you give your Tama the diet soda.
I have done this before. This is what happened to mine:
Your Tama drinks the stuff, you hear the 'evolution' notes playing on the Tamagotchi and see the pixelated flashes and then a mssg saying 30lbs.
Whatever weight your Tama was before you gave it the drink, it becomes 30lbs after the drink. However, if your Tama was 30lbs before the drink - then the weight will not change.
I am going to move this from Comments & Rumours (which is more suitable for Comments about Tama News items or Tama Rumours) to What Happened To My Tamagotchi? - then perhaps we can get some input from other members. They may have had different experiences.
Best, etc.
TamaMum
*Topic Moved from Comments & Rumours >> What Happened to My Tamagotchi?