If it started out as a measure, then it is probably reading it as a number instead of a string. This means that it will treat any non-number as NULL. A fast way to change this is to click on the letters (Abc) to the left of the dimension name and select "string". It looks like this:
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Change the datatype of the column as "String".
This is probably due to type of your Column as Number.
I see you stating that you have default as string but let me show you step by step.
Convert the same to string like below and see ....
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