The default Scheduler for trains with liquids has no abort after finishing.

Trains with Hard cargo have, per default plan, an inactivity in the Depot, which allowes the Dispatcher to kill the request after entering the depot.

Liquids trains, lack this inactivity in the depot so they just run through and keep the request indefinitly.
The Dispatcher in turn keeps then sending more and more Trains after the previous ones run into the Time-Out (default 10min).
Till there is no train left.

This then results in endless train backing up out of the Liquid Provider station,
as X trains try to do the same job, which was already fullfilled the first time.

Now I'm faced with the same problem.

At the station, I request 160,000 units of oil. The train can hold 100,000. The maximum number of trains per station is 1. At first, the station operates normally, but then trains continue to arrive at the full station (200,000) and deliver 100,000 units of oil, even though there is a shortage of 60,000. As a result, I am not sending an extra train to the station, but all of them. and a huge traffic jam is created, requests are deleted after 10 minutes, and the trains return to the depot full of oil.

Although, okay. Restarting the save just solved the problem.