Answer: Based on your description I am not certain what is happening but I suspect it is related to lack of soil moisture and/or too much sunlight. The soil should be organic and humusy, acidic, and evenly moist. This fern, Athyrium niponicum, really likes moisture, the soil should be evenly moist even to the point of having wet soil.
Often, when one plant out of a group does poorly it is due to a rooting problem. It is possible they were rootbound at planting and have failed to root into the surrounding native soil as a result. Or, perhaps there is a large rock underneath them preventing them from rooting downward, or maybe there is a tree root competing with them for water and nutrients. Or, and this is what I suspect is happening since you are seeing that discoloration, perhaps they are in a drier spot than the others or in more sun than the others which would contribute to moisture stress.
Tassel fern or Polystichum is actually a bit more tolerant of drier soil than the Japanese painted fern. I have also found that the Athyrium will tolerate more sun with wetter soil and conversely in less sun can tolerate drier soil.
Since your other ferns are doing well you must be caring for them well. Usually however it is better to water deeply but less often rather than sprinkle lightly every day. That way you will be encouraging deeper rooting where the soil naturally stays moister longer, and the plants will not be dependent on you to water daily.
I hope this helps you trouble shoot.
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