The AI of the class Retro is constantly pitting in races (Guiania, Guapore). 5 to >10 times after the half distance of a 50 minute race.
There was a typo in the F-Retro fuel estimate value which may be causing this. It will be fixed in the next update. Keep in mind for appropriate pitstop strategy it´s recommended to run the actual series, rather than All cars / tracks. F-Retro won´t stop for fuel then.