When adding a filter to a Salesforce report using a record's Salesforce ID, entering a 15-character ID will automatically convert it to the full 18-character version once the filter is applied. For example, if you enter the 15-character ID "001ABC123DEF456", Salesforce automatically converts it to the 18-character case-safe version "001ABC123DEF456GHI" when the filter is saved. This is expected behavior in Salesforce reports.
Salesforce report filters internally use the 18-character record ID format, also known as the case-safe ID. The report user interface displays only the 15-character ID in the column view. Both IDs refer to the same record, and the automatic conversion to 18 characters is by design.
If you need to display the full 18-character Record ID as a column in your report, you can create a Formula field on the object:
To convert 15 - character ID to 18 - character ID, please refer the below article:
https://help.salesforce.com/s/articleView?id=000385066&type=1
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