Informatica power center will act differently when it encounters the DECIMAL values which were derived from different sources like oracle, SQL SERVER.
The real problem occurs when the DECIMAL value exceeds the precision of 25 digits (vary upon the Informatica versions).

Problem:
Consider the SQL SERVER source which has the table employee with salary column defined as DECIMAL (38,0)
Consider the value from the column is 44365263573674236242657456457

If you process the above value, Informatica will round the value like 44365263573674236242000004400 which is undesirable.

Solution:

Enable high precision property in Session level

If the above solution is not working you can go with the below one:

It is better to use String type while reading the decimal values from sources, later we can change to decimal values in Informatica mappings.

Source (Decimal) --> Informatica source (Read as string) --> String to Decimal in Mapping level