Since a few responded to me and other have posted with same issue, dug a bit deeper on missing backpack issue. To recap: After death and backpack drops on death, the backpack is not located yet backpack compass indicator is working . MM suggested using god mode, that didn't work to locate the backpack. Someone else suggested using "LE" command in console.
"LE" in console (along with god mode to get there easy) does help, but the conclusion of this deeper dive into the problem is that the backpack is not recoverable at all. In fact, the backpack appears to be moving. Screen shot in link below showing LE command.
EntityLootContainer must be the backpack. The first coordinate is the West-East coordinate, with negative number meaning West. When standing on the compass backpack icon location, that first coordinate number for the EntityLootContainer is about -1053 with the player pos first number around +360. So travel straight west to reach -1053, but can never get there because when the player WestEast coordinate(first number) reached about -800 the EntityLootContainer first number changes from -1053ish to the current visible number of -2031. And if I keep flying in god mode towards -2031 west, the EntityLootContainer will also change to be -3000 something.
conclusion, the backpack is moving somehow based on how close the player is?
Conclusion right or wrong, there is a definite bug with backpacks disappearing on single player, permanently disappears.
Original death spot (1-2 squares below current spot, this is up in the air god mode:
https://steamcommunity.com/sharedfiles/filedetails/?id=1889746540
Trying to reach the backpack, shows EntityLootContainer has moved if compare POS with first picture:
https://steamcommunity.com/sharedfiles/filedetails/?id=1889735919http://https//steamcommunity.com/sharedfiles/filedetails/?id=1889735919