Data validation list sources referring to tables are only dynamic for tables on the same sheet
Data validation still does not support structured references as list sources. Luckily, the non-structured references to table columns update if the column changes size. However, this only occurs if the data validation cell and the table are on the same sheet. This is further incentive, beyond the usual readability issue, for Excel to support structured references in data validation.
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Bryon Smedley commented
This is quite odd. I prefer to have my validation lists hidden on a separate sheet, but have to use convoluted formula solutions to make them dynamic. I can use a Data Table to make it dynamic, but it can no longer be on the hidden sheet. It would be nice to be able to use structured references for dynamic drop-down lists.