I'm struggling to reproduce it. It's weird, because this was a bug that would happen on a few occasions, but I've since seemed to fix it...until your report haha.
I've even fished out entire ponds and no luck catching it. I'll be pushing an update soon that balances the game out, so maybe this update will fix your issues somehow.
Your picture suggests it's happening while they have their line casted out, so that's where I'm looking.
I'll continue to dig in though and hopefully I can find it.