Filter options are not displayed for a single line of text column in the list view in SharePoint 2013


We have a local document library in SharePoint 2013 with almost 12k items (the threshold of List View was set to 50k items). There is a column with the name Cust Name(Indexed column) of type Single Line of Text.

Filter options are not shown for the column.

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Filters work well for the other single line of text columns (indexed columns).

Can anyone help me with this?