I had to close my tent and open again an hour later just to check my eyes weren't deceiving me and the same thing occurred.
Take a look at the top most growth tips. Yellowing whilst under the light and sealed. Airflow is good enough but I'm guessing my lights PPFD is pretty high. (I'd know if those Amazon bastards sent my meter wouldn't I) considering adding co2 to the mix. Probably a smart co2 bag. (My sister makes them. Search for propa smart co2 I actually made the graphics for that but not something I've ever needed to use so haven't researched properly) tips. Pointers links all greatly welcome here. Could do with an education in this department definitely
Edit she also makes trounce. Which kills everything. It's not really available over here now but if any UK guys need any. I'll be your source. Under the premise it's used on indoor grows only cos cola likes the bees thankyou