Hello Sebastian,
THM50 is essentially used to transfer the ineffective portion of the Hedging relationship. Even though in THM80 it shows as the Hedging relationship as effective, it might be a case that the HR is not 100% effective. If you run THM80 with only assessment it will show you the result of the internal effectiveness test i.e. whether the HR is effective or not. In case you select the option Measurement it will tell you what portions of the HR is effective and what portion is ineffective,
In your case, it might be a case that a small portion of the value calculated is still ineffective which is being posted to P/L.
After running the THM80, if you run TPM1 with Hedge management Log option clicked, the system will show you the 2 portions and their postings separately.
Regards,
Nikhil