The leaves of my plumeria have spots and are yellowing and dropping off. It receives full sun all day . We are in Southwestern Florida.
Look at the undersides of the leaves. Are there little orange spots? From the front, it looks like it might be Plumeria Rust. You can read all about it and see some very good pictures of it here: https://www.ctahr.hawaii.edu/oc/freepubs/pdf/PD-61.pdf
If there are no orange spots, it could be Sixspotted Spider Mite. Look for webbing on the leaves and between the leaves and the stems. You can read more about it here: http://ipm.ucanr.edu/PMG/PESTNOTES/pn74141.html