Issue with social media referers after upgrade to 3.9.1

we just upgraded our matomo instance to the newest 3.9.1 version. Since then we face an issue with the social media referers. According to the piwik_log_visit table, the referer_type and referer_name values are looking different in comparison to the values before the upgrade.
Here is a sketch of the values using pinterest as example data:
(referer_url is the pinterest uk url and same for both)
Saved data after the upgrade:
referer_type | referer_name
7 | Pinterest
Saved data before upgrade:
referer_type | referer_name
3 |
The issue is, that using overview views in matomo, like the main dashboard view, it now shows 0 social media referers for the timeframe before the upgrade. However, if I look into Acquisition -> Social Networks, it does recognise old and new data correctly, if the data is not viewed together (so as soon as new data is within the selected date range it does not include the old data, else it does).

We have multiple websites running on this instance that all show the same behavior. Unfortunately we do not have any other instances for comparison.
Does anyone else face this issue so a bug report may be valid? Otherwise any tips on how we can fix that (possibly without changing the database values)?