I've had some wonky train deliveries this game, and I just caught one of the trains at it. It seems to be caused by the fueling station which is acting like a depot and assigning a new schedule to my trains. Here is the order of events:
Train goes to fill up on coal (this is where I depleted its fuel to trigger refueling)
Full of coal, train heads to the temporary stop at coal unloading
At the temp stop (before unloading) the refueling interrupt triggers and the train diverts to my fuel station
On arrival at the fuel station the schedule clears and the train refuels. The refuel station is assigned to the schedule as if it were a depot, and the train (still full of coal) is assigned a new schedule.

My refuel station is only that, the signals on its combinator are 1. stop is fuel station, 2. encoded network ID, 3. limit trains, and 4. solid fuel 20.
Am I missing something?

can you share a screen shot or a save? The fuel station should only show as a temporary stop and the train should be sent back to the depot after refueling.

Do you have other mods loaded?

While I was recreating a simple save for this, I identified part of the problem.

Here's a mockup. To recreate:
1 - Start the train. It will go load up on coal.
2 - While it is loading coal, remove two stacks of fuel from the locomotive
3 - The train will try to refuel before unloading, and reset its depot to the refueling station name

https://drive.google.com/file/d/1cmzRaHQeymbY22zOOlt1aL3ol2CJnLev/view?usp=sharing

Now, here's how we got here -
For my refuel depot, I pasted in one of my standard LTN depot blueprints (you can see three of them in the save). Then I went to its combinator and deleted the depot signal, and added in the refuel signal to make it a refueling station. Somehow, the refueling LTN station still has a little bit of an idea that it is a depot too. If I delete the LTN station and rebuild it, connecting it back to the same combinator, everything works fine.

will take a look. Thanks for sharing a save

Just wanted to say thanks for the solution, Spacey42. As you said, I just had to rebuild the station and the trains started to leave the fuel station instead of staying there.

https://github.com/hgschmie/factorio-LogisticTrainNetwork/issues/28