gromacs-2021.4: misleading output for gmx trjconv -t0 0

Summary

I am trying to generate a trajectory file for the last 500 ns from a 4 us one and resetting the time to be like a 0 ns to 500 ns trajectory:

gmx trjconv -f step7.xtc -o step7_mol.xtc -pbc mol -s step7.tpr -b 3500000 -t0 0

GROMACS version

2021.4

What is the current bug behavior?

While for the first seconds of run the output is correct:

Back Off! I just backed up step7_mol.xtc to ./#step7_mol.xtc.2#
Reading frame      20 time 3501000.000    ->  frame     20 time 1000.000     

Some seconds later user will simply read this in the terminal:

>  frame    660 time 33000.000          ->  frame    600 time 30000.000      

Instead of the equivalence between the original times and the new ones

What did you expect the correct behavior to be?

It should show the times from the original file and the new ones, as it does during the first seconds of run.