Jaap opened the ticket and wrote:

This case could be reproduced in any game, but here's our game to demonstrate this issue.
At the bottom of the map, we have a Food Processing Plant which produces 27k tonnes of food which is less than 70% transported. The trains at station "Food Load" should transport this food to the other side of them map.

Each train has an capacity of 288 tonnes of food, so this amount is reserved when the train starts loading.
When 1440 tonnes are a available at the station, 5 trains can be loaded at the same time. The other trains are waiting at 0% until more food comes available.

The rating of the station stays lower than 70%, and it doesn't matter what you do to increase this percentage, it's stays impossible. The only way to increase this percentage is to build an extra station and let a part of the trains loading at this station.

I'll suggest to do the next modification in the file station_cmd.cpp at line 3295.
Replace "uint waiting = ge->cargo.TotalCount();"
to "uint waiting = ge->cargo.TotalCount() - ge->cargo.ReservedCount();".
So only the amount of cargo which can't be loaded into a train will be used for calculating the station rating.

Reported version: 1.4.4
Operating system: All

This issue was imported from FlySpray: https://bugs.openttd.org/task/6165