Filled Map doesn't show color gradient

[SSC_STSA]

I'm new to Power BI and have issues with Filled Map color gradient. I have 3 tables, each contains a column for customer zip code.

I created a Filled Map for customer zip code for Table 1 and Table 2, and both map show the color gradient as intended.

Table 1 - column Zip Code is formatted as Whole Number

Table 2 - column Zip Code is formatted as Any (some rows have Null value in the column Zip Code)

However, for Table 3 the gradient shows only one color (dark red). Table 3 is a combination of Table 1 and 2 (appended), so it contains some rows with Null value in the column Zip Code. The Filled Map is formatted the same exact way as the map for Table 1 and Table 2. I tried formatting the Zip Code column as Text and Whole Number but it didn't make a difference. 

For each table, Summarization for column Zip Code is set as Count. Data Category is Uncategorized. I tried switching it to Postal Code but didn't make difference. What am I missing here?

[lbendlin]

Table 3 is a combination of Table 1 and 2 (appended), so it contains some rows with Null value in the column Zip Code. 

Does it also mean that Table 3 may contain multiple, conflicting rows for the same ZIP code?

[SSC_STSA]

Table 3 is Table 1 and 2 combined and a few rows are duplicates (same client ID). However, my understanding is the Filled Map is designed to look only for the column zip code and count them to create the gradient map and that duplicates rows are not relevant. Is this not the case? Many clients also have the same zip code. Could the Null value on some rows be the culprit?

[lbendlin]

I would recommend you test with a smaller, cleaner subset of the data and then add other data points gradually.