I'm not sure that it is a bug, but I certainly had the same problem:

1. The auto-request only sends any given item if a whole rocket can be filled with it — at first, I thought I was doing something wrong, as I wanted to send only the required resources to build the platform as fast as possible ...

To be honest, I have noticed the symbols for the first time as well
I haven't built a lot of two-way rails before, so TIL!

I tried on MacBook M1 Pro with macOS 15.0.1 - couldn't reproduce

I think this is the precise problem:

Before: After:

To me, it looks more like a problem with signal placement, but I might be mistaken.
I've moved some signals around, the train has no problem finding the path, take a look if it solves your problem

Quick update: tested on a clean 2.0.12 version - same behavior, will update the thread name

Blueprint to quickly reproduce:
0eNqlldFu6jAMhl8F+WqTykRKQte+yjShUrwSLU2qJGVDqO8+h8I4R3Ako3NXJ/H3O8rv+ggbM2DvtY1QHUE3zgao3o4QdGtrk9Zs3SFU4GttYMxA2y1 ...

I've tried to do exactly the same on my Windows machine - and couldn't reproduce

I've attached the log and the save file I've just created

Some notes:
- I'm using a MacBook with M1 Pro
- I've created a clean new game and used `/cheat` to get the items
- I've pasted the blueprint attached to the first message - got the same result

Description
When the selector combinator is set to "Sort ascending" and there are at least 8 signals in the network, the returned value of some indexes is wrong (see example below)

Example
Notes:
- on the left side all the selector combinators are set to "Sort descending" - everything is in ...

I'm sorry for creating the bug report before fully testing it, but the root cause really seems to be "placing the train out of order".
To update on the original post:

What did you do?
1. Place a locomotive
2. Configure its name/schedule
3. Place the second wagon
4. Place the first wagon ...

UPDATE: Please read the first response instead, as it provides a more compressed version of it

What did you do?
1. Create a blueprint with a parametrized "train station + train", where the fluid is parametrized
2. Place a blueprint, select a fluid, e.g. water
3. Wait for the train to be placed ...