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.

enter the description of the image here

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

Can anyone help me with this?