Why are records from my data set being deleted after using 'Join' tool

[CDIns]

I have a list of unique values with a category assigned to each unique value, I store this as a lookup table in Alteryx via the Text Input tool. Each month, there are new unique values on my report which aren't stored on my lookup table yet. Why are these new unique values being deleted after the join tool where the category is assigned? Is this a configuration I need to change within the Join tool, or is this inherently how the Join tool works? If so, what would be the best way to create a list of unique values currently not stored in my lookup table? 

Thank you in advance

[alexnajm]

If there are new values coming through in your data that are not captured in the lookup table, they will need to be added to your Text Input. Those records that don't match will drop out of the L/R anchors coming out of the Join tool!

[CDIns]

I see that. So this is inherently how the join tool works? There isn't a way to configure it so that the new unique values just return NULL in that column if they arent included in the lookup table, instead of removing them? 

[alexnajm]

Correct, this is inherent with the Join tool. Add a Union tool afterwards to bring those "dropped out" records back together with the J anchor!

Here's a good article on it: How to Combine Join Output Anchors with a Union Tool (alteryx.com)