Airline looking for a ticket price integration 4 months after the flight

I don’t know if it’s because I fly very often, but I’m surprised by some of the strange things that happen to me with airlines.

This time is the request for an integration for the ticket of a flight that I did more than 4 months ago.

Due to change in plans I had the travel agency adding an intermediate stop to my roundtrip.
It looks like some confusion was made by the agency at the time and that I paid less than I should have.
Now the airline wants the integration and the agency says that this is normal and right.

It really sound absurd to me, like if my car dealer was asking me additional money months after I paid the amount indicated in the signed contract.
Is this really normal?