What you're experiencing is called "damping off". Here's a bit of info on it.

Good luck,
Dewaine

One of the most problematic diseases that attacks seedlings is called "damping-off." This is a fungus disease that could be lurking in the soil or on seedcoats. The disease can even move from one seed container to another by hitching a ride through the water. Damping-off strikes very fast and without much warning. One day your newly germinating seedlings are fine and the next day some of them have toppled as if they were severed by a miniature chain saw. There are several tips to keep this disease under control. Use sterilized soil with good drainage to start your seedlings. Sow your seeds so that there is some space between each seed  do not overcrowd them. Make sure that there is good air circulation. You may have to set up a small fan to gently circulate the air around. Avoid overwatering your seedlings. Sowing seeds into rows will help reduce the chances that all of your seedlings will succumb to this disease. If the disease appears, a row or part of a row can be removed to stop the disease from spreading. Commercial fungicides for damping-off can be used to control this disease.

Do you have a pic? What you are describing does sound like damping off but a 6 month old seedling shouldn't be susceptible. Damping off usually occurs with young seedlings. A 6 month old plumeria seedling should be a decent size.

Thanks for the info, even though it is not good news. Now that I am looking it's happening to a lot of them :( Is there any way once it starts to reverse it?

"Is there any way once it starts to reverse it?"

I start quite a few seeds each year so when i see seedlings that have this condition I toss them. I start seeds in plastic cups so I toss the cup and all.

The few times that I've started "special" seeds and the seedlings grew to 1/2 inch or more in diameter before contracting this problem, I've cut the seedling above the affected area and rooted it with good results.

Best regards,
Dewaine

p.s. here's a link to some images of seedlings with this condition just to be sure we're on the same page.

Here is a link that might be useful: images of damping off

Thank you so much! Yes these pictures show exactly what is happening with them :(
Thanks for the advice, if they are going to die anyways it's definitely worth a shot!