Until 3.2, in order to have a successful sync between primary and secondary, the initial sync has to last less than the oplog window. In 3.4 this is not anymore strictly necessary since the secondary streams the oplog at the same time it performs the inital sync.

The 3.2 constraint on the oplog size is not reported in the documentation, maybe it could be added here:

• Replica Set Data Synchronization
• Replica Set Oplog