Hey,
We're not aware of any bugs with it not showing the weight. I can't remember from playing it myself if the weights only show up when you catch one that's a new record for yourself. Has it never popped up the weight of any fishes for you? If so, then it does sound like its bugged there, unless it's being cut off the side of the screen due to aspect ratio or the border settings in game.
If you go and look at the fishing board in each of the shops, you can see the fishes in the area and if there's a blue ribbon on them, that means you hold the state record for that fish, so that may help out if it is bugged.