Different Result 3vs1 replica set nodes


i was trying to set up a test system to check change stream processes documents correctly, change stream only listens for inserts, and sets some fields based on other collections etc.

the problem is, while i set up test data and trying to create a reliable test, i noticed after processing all change stream data, result of processed data is different when 1 node replica set is used. it is working as intended with 3 node replica set. specifically; a few documents are different in 1 node rs, they dont get updated when a document with one same field from other document is upserted. (i think(?))

i wish i can create a reproducible example but the code and initial data for test is huge, and i really dont know how to reproduce this in a small example, any idea why this happens?