To create an example that triggers the rotation which leads to the bug I did the following steps:

-log something:
$logger a
$logger b

-save the coresponding journalentries:
$journalctl -n2 -o export >> entries.txt

-switch the timestamps and cursors of the entries (result named switched_entries.txt, see attachment)
-this will cause the journalfile to be rotated after the first entry "a" is written

-now follow a directory with journalctl
$journalctl --directory logtest/ -f

-create a new logfile with this entries:
$cat switched_entries.txt | systemd-journal-remote -o ~/logtest/a.journal -

-OUTPUT of journalctl:
No journal files were found.
Apr 27 10:07:40 virtual-fedora-sbs tpiadmin[9617]: a

-some new logging into the file:
$logger c
$journalctl -o export -n 1 | systemd-journal-remote -o ~/logtest/a.journal -

-leads to no new output, however:

-some logging into a new file in the same directory
$logger a2
$journalctl -o export -n 1 | systemd-journal-remote -o ~/logtest/b.journal -

-OUTPUT:
No journal files were found.
Apr 27 10:07:40 virtual-fedora-sbs tpiadmin[9617]: a
Apr 27 14:39:57 virtual-fedora-sbs tpiadmin[11424]: a2
//only the last line is actually new

-this new file will be followed until it gets rotated

-this problem also occurs, if the file gets rotated for other reasons than this "errors", for example if simply a rotation due to size happens



-some weird interaction I noticed: If the logs are put into the journal file two times:
$ journalctl -o export -n 1 | systemd-journal-remote -o ~/logtest/b.journal -
$ journalctl -o export -n 1 | systemd-journal-remote -o ~/logtest/b.journal -
-then the output of this doubled input is shown once, however the file isn't properly followed after that (new entries won't show up)