September is Abbreviated to Sept even though Left() function is used

Remove the 3 from the text() function. The text() function only needs:
The date value (ri!date)
The format string ("mmm")

if(
 rule!APN_isBlank(singlePV: ri!date),
  "",
  left(text(ri!date, "mmm"), ri!format)
)

This is still not working. As the left function needs the number of characters to be extracted from the text which is still missing from the code. It is not giving any result.

May i know what input and output you are expecting to help you better?

I am expecting incoming date in the format "09/09/2025" and output should be in dd-mmm-yyyy. For rest of the months the existing code is working fine. However, returning September as Sept.

Input - "09/09/2025"

Output - 09-Sep-2025

Could you try this once
Let me know if that works for you.

text(
  index(todate("09/09/2025", "MM/dd/yyyy"), 1),
  "dd-MMM-yyyy"
)

You can simply use text to get the desired output, why make it complex with left(). For every date this will work for you by passing the expected output format in text(). 

text(ri!date,"dd-mmm-yyyy")

Harsha Sharma  Not working still 

Harsha Sharma  Not working still 

Can you check if under Settings->General you have locale set as English(United Kingdom). Looks like the Internationalisations settings are at play here causing this behaviour. I have it as United States and it returns output as 'Sep' 

If for reasons, you cannot update the Locale to US then you can try and handle the September month like below. Give it a try!

if(
  month(ri!date) = 9,
  substitute(
    text(ri!date, "dd-mm-yyyy"),
    "-09-",
    "-Sep-"
  ),
  text(ri!date, "dd-mmm-yyyy")
)