Without being certain it looks like it could be due to stress possibly from very cold conditions or due to the growing Media which may be suffering from nutrient deficiency did you amend your soil with compost before you planted your seedlings?
I made a video about growing broccoli from seed you might find it helpful
They were planned from starter plants. Soil is good garden soil with black cow turned in. It has been unreasonably cold this winter. Perhaps I can try again.
Looks like they did get stressed and flowering too soon. Mine are about to go into the ground, been inside all their lives..I will harden them up a day or two first. Some broccoli but mostly romanesco and Brussels sprouts. I hope it stays cold enough for them.
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