Hello @Patrick_Timothee,
Thanks for raising the concern. We have not heard from you in a while, could you please confirm this issue has been resolved for you?
Partition Sync does not take away relationships. It’s defined differently on the cloud i.e. in MongoDB. You can read more information on Realm Relationships here.
The difference between Flexible and Partition Sync has also been highlighted in the bytes.
Note: There may be a few changes since the time the topic was posted but the general idea is still the same.
I hope the provided information is helpful.
I look forward to your response.
Cheers, 
Henna