I was printing a tray about 10 x10" sq. The first layer was about as perfect as it could be but the second layer was crap. Under extrusion maybe, there where gaps between the rows of printed material and they were not sticking to the first layer very well. I let it continue and the third layer was better, not counting the ruff surface caused by the previous layer. Each layer was better still and by the last, Fifth, layer the finish was OK except for a couple of marks. Any one have any Idea why an “in between” layer would be bad and the rest is OK.
P.S. the bottom had a good finish.
It is a bit vague as it could be a large amount o things. I would look to flow rates. It is a large area print and it is possible (guess) that the extruder is over taxed and not absolutely melting the material. this would resolve as infill and lower volume layers no longer need such maxed material. You could do flow rate tests.
I found my Bambu to be very poor and to print well and strong it needed to be slowed 50% until I replaced the nozzle with a high flow.
It did look like it might be the flow rate but why would the layers before and after print OK, printing over a bad layer not withstanding.
Could also just be gremlins. I hate gremlins! 
don’t we all. They always seem to find their way back after being kicked out too. Gremlins…