Over-watered and/or over-fertilized is a potentiality no one could argue against, given what's known, which really isn't a whole bunch at this point. How about an image of the entire planting, including the pot?

The usual suspects would be: Over/under-watering, root congestion, a high EC/TDS level (too much salt in the grow medium/ soil solution), a badly skewed nutrient ratio in the grow medium due to the soil not being flushed regularly or effectively, natural senescence - depending on where the spoiled foliage is located on the plant.

Sometimes a valid diagnosis can be reached by the process of elimination. Other times, you might find it easier to simply take a good overview of all cultural conditions you hold sway over and make your best attempt to dial them into the plant's sweet spot.

Al

if thats the thermostat on the wall .. then thats an interior wall.. and that usually means there are low light issues ..

and in this case.. it seems like the oldest leaves.. which ALWAYS fall off ... annually or cyclically ... are going dormant dead.. perhaps earlier than usual ... due to low light ...

if it were mine.. i would propagate some new pieces.. and when those got going.. i would take momma.. break the pot apart.. get the variegated one in its own pot.. and then get rid of the old plant.. since i would have a bunch of new smaller plants ...

ken

Eliminate those bad leaves and try repotting. Could be root rot from over watering, some disease or a combination. When you clip leaves you must use alcohol on blade after every snip to keep it from spreading. My girls are in 8 inch pots so i water every two weeks with a bottle of and a half of spring water. I pluck off every dead leaf and I use neem oil to ward off bugs.