You mentioned that the seedlings were fine a week after transplanting and so I suspect this is a case of fungus called 'Damp'. In ideal (wet) conditions they appear overnight and kill new plants fast. Can I ask if the leaves that sprouted are true leaves? or simply the green looking cotyledon(I think this is the term) which are really part of the seed and not true leaves? Normally you need to wait for true leaves to appear before transplanting. Earlier and the plants are more susceptible to diseases.

When you transplanted them, did you provide regular venting and air circulation. Both help to control fungus growth.

Get 'No Damp' w/c is a fungicide used to treat seedings or an equivalent fungicide used only to treat seedlings. Although, if all were affected and killed off already, it's probably a good idea to start fresh and to use new & sterilized materials.

This doesn't sound like damp-off, which is a fungal problem where you'd see the seedlings fall over or they would have blackish stems near the soil line, then die.

It may be a cultural problem in some way. Brown tips from your seedlings could be caused by overwatering or underwatering, too much fertilizer/salts, poor humidity, flouride in water, and poor humidity. If they are too near a strong light, the newly fragile leaves could be just getting burned.

Do you have a photo of your setup? The climate for your seedlings has changed because they are no longer in a biodome, and chances are the light has changed, too. What light source are you using? Have you used fertilizer? Side note with zinnia-use a small fan to circulate air near seedlings, not on top of them, to prevent that damp-off, but I think this is something else.

I agree that it doesn't sound like damp-off since it is the leaves not the stems, correct? Leaf tip-burn is almost always caused by over-fertilization or sun scorch. As tuscanseed said something has changed to cause this. What potting mix did you transplant them into? Direct sun exposure? Too close to lights if using them? Or too much fertilizer?

Dave