TheGamingPc wrote on Jan 20^th, 2019 at 2:57pm:

Hello, I've had the exact same issue on 1.300 yesterday, I updated to 1.301 today to see if the issue with fuelling was still there and it was, no matter how much fuel you put in via the FMC or the default P3D fuel and payload system the fuel momentarily gets reset back to 100% I've not encountered it ever before. Thanks. :)

Are you sure the P3D4>OPTIONS>UNLIMITED FUEL setting is OFF?