Your mature ivy is suffering from two problems: too much direct sun and severely dry soil despite your watering routine.
Six hours of "morning sun" likely includes intense midday rays. Ivy thrives in shade or bright indirect light. Move it to full shade immediately. Remove all blackened leaves and withered stem bases.
Water running straight through the pot means the soil is hydrophobic (repels water). The roots are bone-dry.
Rehydrate properly:
Your ivy will bounce back vigorously in shade with consistent moisture.
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