PIC18F44K22: Can't use MCLR/RE3 pin as input. MCU resets when RE3 is held down
If you haven't disabled "Single-Supply ICSP Programming" in your config bits (CONFIG4L.LVP) then this would explain the behavior you're seeing.

Section 24.9 of the datasheet reads "While in Low-Voltage ICSP mode, MCLR is always enabled, regardless of the MCLRE bit, and the RE3 pin can no longer be used as a general purpose input."