This is consistently reproducible behavior when trying to symbolize use joined fields using CSV. Using XSLX is hit and miss.
Solutions
1. As Dan said, create a new feature dataset with the joined data included.
2. Since that isn't always desirable, "all" you have to do is select the joined field you want to symbolize and if it doesn't appear, click the Set an Expression button right next to it and either add zero (+0) or subtract zero (-0) at the end of your field. No actual change to the values, but it suddenly works.
Here's a video demonstrating: nosymbologyjoinedfields.mov - Box
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