Sorry about your frog. I constantly worry about my own husbandry. Humidity is often very hard to control, unless you mist often. I cannot say for sure what caused your frog to die. It may have been a defect. It does happen sometimes.

Ideally you should have cooler night time temps, I have my frog's thermostat set to 76-78F at night and 84-85F during the day. There have been days that have made it hard to control the temperature, however I would usually improve the situation by removing the container lid, and add extra water to the substrate to increase evaporative cooling. Also I have found that dechlorinated ice added to the water dish and placed on top of the container helps get the ambient temperature down. In an ideal situation you would place the frog's container in a cooler area in the house and keep it heated during the day. This is the best way to maintain a temperature gradient.

Otherwise, I should assume that the area you kept your frog in wasn't ridiculously busy, nor prone to vibration or other bizarre stress causing circumstance.

How did you supplement and what did you feed him? Because from January, to now, it defiantly is strange to be so small.

I suspect you may probably chock it up to a defect.

In the end, should you choose to get another frog. Good luck, hopefully it doesn't go wrong next time.