I don't remember this happening before, any ideas about what this is and whether I should worry about it?

I wouldn't worry. But you may as well pinch off the bad leaves, they aren't doing anything for the plant once yellow, and whatever dwells in necrotic spots of any cause is just as well removed from your seedlings.

Couple of causes of spotty leaves in seedlings under lights:
1) happened to get a bit of dirt on it that didn't fall off. Something unfriendly to leaves then started to grow under damp dirt.
2) overcrowded leaves can get condensation forming on the leaf surface. This can either mold or lens when exposed to light, spotting the leaf.
3) edema may have made some blisters on the underside that weren't noticed but afterwards become necrotic spots.
4) I notice the cotyledons have turned yellow as well. This suggests to me the plant hit a low N, and decided to sacrifice the cots for their N. Same goes for lower leaves, the plant will sacrifice the oldest leaves to pull that N for other uses. Immune defenses are turned off when a leaf is being recycled, generally, and so they often succomb to whatever form of disease they are susceptible to, as well as turning yellow. Presto spots.
So I would remove the sickly leaves, and feed the plant some ferts.
The plant may also draw N from cots and lower leaves if the upper canopy is so dense they aren't getting enough light down there to be of service.
In this case, if N shortage is not an issue, the plants just need more space around them to let light in. They're ready to be planted out or potted up!

Thanks, I'm two weeks or more from plant out, and I've already potted up, so I'll feed them some more. Should I still be using half-strength fertilizer? Or is there a point where you switch to full strength? I've been using the Neptune's Harvest Tomato and Veg.

IDK about the ferts, someone else will answer that.
I have a similar question regarding the tomatoes I started for the farm, which have lost their cotyledons and are waiting to be potted up and put out in the light, but IDK when that will happen as my friends are in the process of farmhouse renos before the big move. They are in chunky sixpacks (which fit 36 cells in a 1020) and I am wondering if I should start feeding them liquid ferts - although what they really need is more space and more dirt!
@Setec Astronomy if you get some nice days, the plants will appreciate if you can put them outdoors even for a few hours.