I doubt that what we are calling phantom drain at 80% versus 100% SOC is really any drain but an artifact of Lucid's SOC estimation. In other words, voltage changes between between just before charging stopped and after it has been sitting. Just before ending charge at 80% there are still voltage drops due to current flow and internal battery resistance that have to be accounted for. The battery module temperature may not be be completely uniform so the voltage correction for temperature may have errors.
When approaching 100% charge, the charge rate decreases to protect the battery from damage that would occur if overcharged. Hence, the current decreases and the resistive voltage drops decrease. The battery module temperature may also become more uniform since less heat is being generated. This results in a better SOC estimate that does not change after charging stops.
In any case, Lucid software should not be be making these mistakes in SOC estimation so we should report it to customer care for a future software fix.