I had the same thing happen in DWC, personally it didn't have any impact on growth and it didn't continue developing, but I researched and came to the conclusion that it could be 2 things:
1_ Excess light, it could be light burn.
2_ Something related to calmag, which coincidentally is often linked to excess nitrogen, meaning if there's excess nitrogen, the plant won't properly absorb calmag, which could show a deficiency due to nitrogen excess. If the plant's leaves are very dark, I recommend lowering the nitrogen dose in the fertilizers that contain it a bit. If the leaves are a normal color (as I can see in the photos, although I don't know what color of light they are under), you could add calmag in low doses to see what happens, although if it's your only symptom I don't consider it a problem.
Anyway, I raised my lights a bit, added some calmag, and the new growth was normal without issues, it's not something to worry about, cheers!