Hey there I had a plant do a similar thing...1 yellow leaf. But as for the 1 leaf I wouldn't worry. If more show up then there may be a problem. My yellow leaf appeared, stayed that way, but was followed by good positive growth. Eventually I removed the leaf as I figured it probably wasn't fully doing its job after getting ruined.
Your plant looks great and I love the lst style you've done. I hope she turns out perfect for you.
Happy Earth day! :)