Use of to_timestamp causes full scan

Has anyone seen any performace problems with the use to to_timestamp?

When I use it in a where clause I get a full file scan, when I  don't it
uses the index
for the query. The begin_time column is of type timestamp.

This does a full sequential scan
    select id from details where begin_time > to_timestamp('03/08/25
18:30');


This uses the index
    select id from details where begin_time > '03/08/25 18:30';

Don