I am thinking that a good test might be to create a new database and load a copy of your database into it ...

Then load a clean copy of Zen Cart v1.3.7 into a subdirectory such as:
/test_shop

Then copy your 2 configure.php files
/includes/configure.php
/admin/includes/configure.php

Change the paths and database name, username and password to match this new directory and new database ...

Login to the Admin and change to Classic Template ... and turn off the setting for SEO as it is not in this code ...

And see what the Sale Prices do ...

Something appears off on your site and I don't know if there is code out of date, an add-on or what affecting things as this isn't making logical sense based on what you have said so far ...