Joins between datasets not working correctly with Dataprep on GCP

[Levi_]

I have two data sets, one contains registration details for my users and the other contains transactions for my users. I've done a join between these two data sets with user id as the shared key so that the transactions dataset can have the registration date for each user. The results are very strange. For example, customer 1234 who is on both datasets has the registration date gotten from the transactions dataset on some rows but on other rows the customer 1234 is missing the registration date. I'm seeing the same behavior with other joins as well. Is this a bug with dataprep or am I missing something?

[Levi_]

Additional information : These strange results are on the output of the recipe in my case big query.