systemd's journal is designed to prevent this kind of tampering.
The journal file format is documented here and it describes its support for in-line Forward Secure Sealing:
Tag objects are used to seal off the journal for alteration. In regular intervals a tag object is appended to the file. The tag object consists of a SHA-256 HMAC tag that is calculated from the objects stored in the file since the last tag was written, or from the beginning if no tag was written yet. The key for the HMAC is calculated via the externally maintained FSPRG logic for the epoch that is written into epoch. The sequence number seqnum is increased with each tag. [..]
see Tag Object
What you can do is clear the journal with the log entries from the time before you fixed the service (e.g. you fixed it two days ago):
journalctl --rotate --vacuum-time=2d
Source: How To Clear The systemd journal Logs