That's Catharanthus roseus, commonly called 'vinca'. I can tell you from a great deal of personal experience that this plant can thrive in the heat, humidity, blazing hot sun, reflected heat and sun, etc. It's a common parking lot flower in locations where little else will do well. Catharanthus almost overused in the deep south (where I lived for over 20 years).

SO! Heat and humidity are not the problem. Over watering and poor drainage can be issues with this plant.

Here's the bad news: Catharanthus can be susceptible to a few fungal disorders. These fungi exist naturally in nearly all soils, but only become a problem when a susceptible plant AND just the right conditions come into play.

With phytophthora, rhizoctonia, and alternaria, overly moist soils can be the instigating factor. The darn plants will do just great in that blazing parking lot island with no irrigation, but expose them to a bunch of rain for several days.....Bingo!

I soon learned that Catharanthus is also a signal plant for nematodes, sad to say.

After viewing the pics, I'm sticking to my story of probably too much water/overly moist soil, as mentioned in my other reply.

The wood chips might be keeping the roots too wet for this plant and perhaps the soil is compacted and not aerated enough. Judging by the foliage color,the plant is not taking up nutrients from the soil,usually a sign of root problems.
The pH might also be off.

Yellowing and stunting are just a couple of the symptoms of root diseases OR nematodes.