Mandevilla leaf drop
My Mandevilla plant has beautiful and plentiful red blooms. Suddenly I find lots of green leaf drop, not yellow. Plant is in sunny area and is in an outside planter with the bottom cover that snaps in. This prevents dirt from flowing through the plant when it rains or is watered. Could the water in the base be harmful to its roots. Container is too big for me to empty after rain showers or watering. Should I remove the base which holds sitting water?
Since there are a number of plants that drop green leaves when the roots are staying too wet, that's a good guess. In my experience, the soil doesn't really wash out of the container, so it's not necessary to have a dish there to catch it. Definitely try removing it to see if that improves drainage. In order to find out if the soil is actually too wet, you can test it with a moisture meter, or simply stick it with a bamboo kebob skewer, or some such stick, as if you were testing a cake. You can feel with your fingers how wet the soil is. Make sure to check all the way to the bottom of the pot.
If you've had the plant awhile, the problem could be that it's outgrowing the pot. No more room for roots means it has to get rid of some leaves.
Also, it might want a little more shade. Sometimes I move plants half a dozen times before I find them their "happy place." Here's a good article on mandevilla: https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm