Why are my Acer seedlings drying up and dying?

Question

I’ve grown lots of little Acer seedlings from seed I picked last autumn and germinated early this year after 100 days in the fridge. I was very excited about the success of the seeds but now at a good enough size they are dropping like flies. They were on a sunny window sill and fine through April’s heatwave but then suddenly in July started getting shrivelled browning leaves and slowly dying. It’s barely been hot June & July but in case it was the sunshine I moved them to a north facing window and they are still dying. They’re in general purpose compost with a bit of perlite in well draining pots. Is it purely that they need to be outside and not in a house environment? We don’t have the heating on. Some are thriving but the most successful one I’ve potted on and now it’s looking very unhappy!