Line level scripts on events like post-read, pre-write or post-write tend to cause this because of the way the UI object tracks movement between rows in a grid to determines if a line write is needed if the line you came from had been edited. 

I believe this has come up a few times before but mostly on the pre-write event because column level events don't cause this.  I believe I have posted a few different ways to get around it, one would be to figure out the last row and use the ShowRow method on it, another one would be to delay the setting of the value by using the CMD_STR$ variable.

This thread has some examples.

(+) Strange Behaviour when Setting a Date UDF from Button Script - Business Object Interface - Sage 100 - Sage City Community

Here are additional threads that include some of my posts on CMD_STR$.

(+) Changing a date field on a new line with an unrelated Pre-Write script also changes that date's format - confuses customer - Personalization, Customization, and Productivity Tools - Sage 100 - Sage City Community

(+) 2016 - 2020 First oScript.SetError Method doesn't trigger an error message in Lot/Serial Number Distribution - Personalization, Customization, and Productivity Tools - Sage 100 - Sage City Community

(+) Script advance to next line - Business Object Interface - Sage 100 - Sage City Community

Line level scripts on events like post-read, pre-write or post-write tend to cause this because of the way the UI object tracks movement between rows in a grid to determines if a line write is needed if the line you came from had been edited. 

I believe this has come up a few times before but mostly on the pre-write event because column level events don't cause this.  I believe I have posted a few different ways to get around it, one would be to figure out the last row and use the ShowRow method on it, another one would be to delay the setting of the value by using the CMD_STR$ variable.

This thread has some examples.

(+) Strange Behaviour when Setting a Date UDF from Button Script - Business Object Interface - Sage 100 - Sage City Community

Here are additional threads that include some of my posts on CMD_STR$.

(+) Changing a date field on a new line with an unrelated Pre-Write script also changes that date's format - confuses customer - Personalization, Customization, and Productivity Tools - Sage 100 - Sage City Community

(+) 2016 - 2020 First oScript.SetError Method doesn't trigger an error message in Lot/Serial Number Distribution - Personalization, Customization, and Productivity Tools - Sage 100 - Sage City Community

(+) Script advance to next line - Business Object Interface - Sage 100 - Sage City Community