Hi Krunal. The contents of the document are somewhat sensitive, so I tried copying the table to a new document (with your great new copying features!). While doing this I found out what is wrong.
The very first visible column that is of type Text, Date, Number and not a display column would show up as “subtext” column inside details’ list selector.
However, currently it seems only “Text” columns will show there. Date or Number columns will work if selected as display columns, but the subtext seems to pick the first text column in the order shown in the table.
I actually needed a date column to be shown as subtext, but I worked around that by making a formula-based text version of that column and placed that first - and the results work perfectly. This is something I need to do anyway, since it seems that Coda does not respect the selected formatting for the date column (YYYY-MM-DD), but instead uses the horrible non-standard way to represent dates (M/D/YYYY) if a date column is made a display column - so I am using a formula to enforce that the string version has the correct format.