This looks like Plumeria Rust Fungus. Since it's a fungus it has spores & is highly contagious to your other Plumerias! (The leaves/spores are even contagious on dead dried out leaves too!) You need to remove the diseased leaves & dispose of them immediately. I highly recommend using a Copper Fungicide. Lowes & Home Depot both carry "Bondie 775 Copper Fungicide". (The concentrate is a much better deal & then you can put it in a more effective sprayer of your choice too!) It is an organic fungicide, works on a wide variety of plants & fungi/diseases, & most importantly works well as a preventative measure as well!

https://plumeria101.com/plumeria-rust/