looking at the documentation for FillNtupleDColumn, I am not sure if the way you call it does what you want:
G4bool G4RootNtupleManager::FillNtupleDColumn ( G4int ntupleId,
G4int columnId,
G4double value
)
ntupleId is 0, which is the first ntuple - should be fine
eventID presumably can be a very large number, and this goes into the columnId in your code. Could it be that all columns before these events are filled with zeros? So at the end you get a single ntuple but a large number of columns. Instead, you want a large number of ntuples, with 1 column each, right?
I would try to change the line into
analysisManager->FillNtupleDColumn(0, Globaltime);
and see if that helps.
It’s also often helpful to check the examples and how things are done there, e.g. geant4/EventAction.cc at geant4-11.0-release · Geant4/geant4 · GitHub