You didn't mention any type of fertilizer being used. It could be a nutrient defieciency, but we can't tell you for sure without a picture.

Here's the most prominent

Probably just an adjustment to the new light conditions

Also some curling of the leaves and yellowing around the edges.

Same...lemons are sensitive to light changes

Please post a picture of the underside of an affected leaf.

What the heck kind of potting medium is that? That could be part of the reason for your problems.

Looks like moon dust.

Hello Isabelle Fleszar, i think you could try prolonging the lighting time to 12 hours, and try fixing the lighting height to 1 foot/0.3 metri. I recommend SANSI Led grow light bulb to you, you can see more details on amazon.

The leaves are losing moisture from the FAN, low indoor humidity, root temperature and lack of sun. In theory the fan is a good idea. Plants need air circulation. Indoors a fan blowing directly on the citrus will quicken evaporation. The leaves are starting to dry out. A clue is the curling leaves. Misting with warm water would help. Amazon has some very inexpensive grow tents. Ideally citrus want 70% moisture. I can see from pictures the dryness is taking a toll. Maybe move plant into bathroom. LED light is a great idea. Catch22 though as most have UV spectrum which can also dry leaves out. Find a balance between not too close and close enough.