I've got a bunch of McColley's Finale and Prince of Orange philodendrons at my work that are almost all displaying this same issue. The leaves are turning a dull brown from the outer edge in and getting a floppy leathery texture. I've tried drying them down, feeding, and treating with fungicide to no avail. I really don't know where to go from here and I hate to see these plant not thriving. It's been going on for months now. I've attached some pictures. Any advice would be greatly appreciated.

The plants are very small and are potted in pots too large for them that allows the medium to star wet under the surface too long. I suspect your plants have suffered root compromise