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Query #1: ClearFiltering function does not work correctly.
We can able to reproduce the reported issue when we call clearFiltering method to clear the filtering from more than one column. We have logged a defect report for “clearFiltering method will send multiple posts in excel filter” this issue. The fix for this issue will be included in our upcoming April 17, 2019 patch release.
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Query #2: By the way, isn't it possible to “teach” a grid to determine the data type of columns not by the first record, but on the basis of a model?
By default Grid will set the data type to the column based on the first cell value. When the first cell has null value, you must need to set the date type to that column by manually. This is default behavior of Grid.