Re: NPCs in fishing career without owning IL

There's always been a base game NPC only fisherman career - NPCs with that usually also have the trait "isMasterFisherman" and they're usually assigned this to be background fishing people you'll see around the world using fishing spots, usually wearing professional fishing clothing like dungarees. That's how you can tell them apart from just regular sims fishing.

Like your screenshot, it will show the sim as a "Fisherman/Fisherwoman" working for "Fishers and Sons". So I don't think you have any bug. NPCs with this career will have the career stripped from them if you ever make them a playable sim, because it's for NPCs only.

The part-time fisher career from Island Living is different - you can tell those sims apart because it will say they are "Fisherman/Fisherwoman" at "Gilbert's Guppy Market".