time not flowing the same is a common enough trope in isekai series, but it's usually both move forward at varying rates...but this, this means time sometimes flows REVERSE between the two worlds, as the first book in the lord of the rings series was released ~9 years after WW2 ended
...I mean, it is possible it time skipped more than a decade and the skeleton showed up after the soldier, but that seems unlikely...plus how would the Japanese soldier read english? unless otherworld translation magic works on languages not from it's own world too somehow? far less likely than some other Japanese future person having a translated JP version and reverse time flow