Hi Ponilango,
Yes I've confirmed that MII 12.2.3 will allow a string variable to be used to with an addition operator in the expression editor. I created a simple transaction with an assignment action that took a string variable + 1. It works fine, however, in 12.2.5 it will throw a conversion exception.
We have found ways to fix it - converting to numeric data types,initializing the string variable to a number, possibly using the number function to convert. These are all good ways and in most cases probably the "right" way to handle this. However, I'm trying to avoid doing a review of the entire solution and all of the testing that comes along with making these changes.