Where did you get the 64GB logic board; do you know for certain that it worked perfectly, without a battery drain issue?
If you don’t know for certain, then there may have been an issue with the 64GB logic board (and probably why is was available for sale in the first place).
If this is a logic board you owned and are certain of it’s condition, then I would double check your work and look for tears in the flexes or damage to the connectors. Another thing to try is to disconnect all of the connectors except the battery, Dock and screen (LCD & Digitizer, do not connect the FCAM & Sensor). Fully charge the battery and then test how long the phone lasts with full display brightness and the screen not turning off. Compare this to a similar setup with the 16GB logic board, they should be the same, or close enough. If the 64GB board works just as well, then add one flex at a time until the problem reappears and replace the defective component. I would start with the Home Button, Rear Camera, Volume & Power Button, Front Camera and Sensors.