Hole in middle of tomato seedling leaves — thrips?

[Prancu]

This is my first time growing tomatoes, and while they do seem to be growing, most of the seedlings have a hole in the middle of at least one leaf. It starts small and grows bigger until the leaf dies. It's not actually a true hole, though. It doesn't go all the way through. I know there are some insects that do this, but these are inside, have never been outside, and are grown from seed. I also sterilized the soil before I planted them (never again, fungus gnats!). So I don't know how insects would have gotten to them. However, I was having a different issue with my peppers (looked completely different) and people on here identified it as thrips. So I guess anything is possible. I have been treating all my plants for thrips, just in case, and I've never actually seen a single bug, but could this be thrips too? Or another pest? Or something else? For context, these are under a grow light for about 15 hours a day. They have a fan on them during that time. They are also still on a heat mat since it's been cold inside. I also have a ton of other plants and none of them are having this issue. I'm attaching a picture of one of the leaves. Sorry it's kind of blurry.