Nutrient burn? Fungal? Weird strip "suffocating" autoflower seedling

Hey so I'm new all this, would appreciate some help! Potted 3 mephisto autos around a week ago in 3 gal pots. Used a 1:1 ratio of old potting soil and mushroom compost with a bit of perlite. My other 2 autos seem to be doing fine right now ( about 7 days in). But one girl is showing this really weird 'strip' of discoloration and one leaf, and a sort of nutrient burnt look on the other. Over the past couple days both the strip and the burnt look have gotten more pronounced. It almost looks like on the strip side like a rope is suffocating the leaf- the ends getting smaller. Anyone have any clue what this could be? I've been doing research into nutrient deficiencies/ pests and I cannot find anything like this. Really want to know if this could be a situation where I need to isolated her from the other girls- which I guess in that case it would be fungal? Also would love to know if there's any hope for the lady and what to do to help her thrive. Thanks a billion!