Sorry I forgot to answer.
The bug with the rail colors unlocked is a new Bug.
The "bug" where the train is not returning to its origin (same colored splitter) is a known issue, however we consider it not-a-bug for the moment.
https://discord.com/channels/1000343719314198548/1274668219726036992
The train will always use the shortest route on the same colored rail.
If you want to split them, you can use different train colors.
Many players had the same problem and a different expectation.
So you are absolutely right, it is not communicated well at the moment.
Trains are still being worked on during the early access phase and we had some players demand more colors for trains to help routing.
I hope this will solve the issue in the future.
To sum up, trains are sticking to the color path, if the path splits, it may not follow back to its Train Deployer Platform and instead follow a different same-colored path.