No idea what the problem is here but the code definitely works and works on the Mega2560.  I tested with a Mega2560/RAMPS1.4 if using a MKS Gen-L be sure you pick a pin for 1-Wire that has no extra filtering circuitry (no end-stops.)

Here's the logging output (#define FEATURE_LIST_DS ON) and the result of an :Ft# command to get the focuser temperature.

06:45:23.865 -> DS18B20: 0x286488AD080000E5 auto-assigned to TELESCOPE_TEMPERATURE
06:45:23.932 -> DS18B20: 0x28F2B7AB08000004 auto-assigned to FEATURE1_TEMP
06:45:24.000 -> DS18B20: 0x28B571AD08000043 not assigned
06:45:24.033 -> DS2413:  0x3AD836460000000B auto-assigned to FEATURE1_PIN
06:55:45.581 -> 23.3#