Photos below.

The last two were from a week ago when most of the seedlings were doing well

added details: I'm starting them indoors, I don't have a place outside that's sheltered enough for them, and my apartment is about a constant 70-75, going down to 65 at night. they get plenty of light but no direct sun.

I'll give my opinion based on growing a lot of Japanese maples from seed along with other trees.

1) You need a lot more light, either sun (they can take more than you think) or supplement with more light when growing indoors (overhead lighting for example). The seedlings are tall and leggy.

2) They are likely falling over and dying from damping off (google it for more info). I go through this every time I grow maples from seed. A couple of things you can try to limit it. First, make sure you don't over water. I had mine outside and we had record amounts of rain earlier this year so that didn't help. Second, make sure you have a well draining growing medium. Third, you can use some sort of fungicide to control disease. I usually go a more "organic" route and used Actinovate. It seems to help as I didn't lose anymore trees after applying it, though it is hardly a controlled study.

Thanks a bunch, fungal issues definitely makes sense, especially if I've been overwatering and not giving enough sun.

They are too warm for the available light. Next time germinate them outdoors. It's a lot easier. It's a little late for you to move the ones you have outdoors. Most look too far gone. If you do, put them in bright shade for a week or so before moving them into more light. They appreciate relief from the afternoon sun in almost any climate. Where are you?

Northern California on the coast. It's normally a near-perfect environment and there are tons of maple nurseries around.

My issue is that my balcony is extremely exposed, I've had at least one larger maple get scorched on it before I brought it inside for the summer.

At this point I'll be lucky if 2-4 of my original 20 survive, but this is the first time I've tried to grow them from seed.