Trouble adding to 1 to Many relation in the Users data table

There seem to be similar questions asked, but none of them have been sufficient in solving my problem. My problem is that updating a 1 to Many relation in the Users table SOMETIMES works. My code is as follows

[user setProperty:@"conversations" object:@[conversation]];
Fault *error = nil;
[backendless.persistenceService save:user error:&error];

“conversations” is a 1 to Many data object relationship.

After executing this code, the conversations property has only 1 object, the conversation that it was just set to. The peculiar part of this code is that it works some of the time and for some users, but randomly. I executed this same piece of code for two different user objects, one after the other. For one of the users, the new conversation was added to the relation, resulting in 2 conversation objects in the “conversations” property. For the other, the first was overridden, resulting in only 1 conversation object in the “conversations” property. I am very confused as to why this is happening and I would appreciate some help.
In the question I have supplied all of the relevant code.

My APPID: C1C73839-75FD-DF58-FF72-2CBCEA0F9100

Hi,

To update User object you should retrieve this object from Backendless with relation you want to update( use relationDepth in find query, for example ). After that you can update/delete existing relations or add new.

Regards,

Denys