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

When inserting code using this is recommended Insert->Code

if(
  rule!APN_isBlank(singlePV: ri!date),
  "",
  /*left(text(ri!date, "mmm",3), ri!format))*/
  left(text(ri!date,"mmm"), 3) 
)

text() needs two parameters (value, format), left() needs two parameters (text, num of chars). Your text() has 3 parameters which might be causing this. Try and work with the above code to resolve the issue. 

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)
)