so i test with sponge and with out sponge, and it seems items are kept on death and i have no clue how to fix even put /gamerule keepInventory false and still happening.
One thing to double check, is what world you are in when you are doing the gamerules.
Strange that everything seems to be set to keep inventory true though since that isn’t the default.
What plugins / mods / SpongeVersion are you running?
gravestone mod was making it dupe items had to remove it even tho i would like a fix for it, people on the github posted about it.