Well, this took a LONG time to reproduce the issue after your latest fixes... but here we are.

Save:

https://disk.yandex.ru/d/T2N54RAArC4dxg

Where:

On 11 hours (2nd big island directly to the north), there is a large facility producing rocket frames & space science.
Here, find a cargo port for Rocket Frames supply chain (the top-most cargo port, with 10 train stops).
ALL red inserters containers into trains there are stuck. Ones unloading trains are fine.

Port name: "[item=port] [item=ff-rocket-frame] Production Chain: Rocket Frames: Production"
(can also be found via trains & stops menu).

I have a couple of ideas of why but I need to check them all, Also I still have not reproduced it for now.
Since it's late for me I'll check this in the morning, I have a suspicion about left/right positions but well see.

I suppose there's something related to the switching of train automatic/manual mode. Because that's the only thing I changed for trains after their 1st initial (and successful) load.

Wait this happened without changing the inserter pickup/drop position?

Wait this happened without changing the inserter pickup/drop position?

Yes.

Then most likely this mod has nothing to do with it.

If you see from the point of view of factorio the only thing that this mod does is changing the pickup/drop vectors of an inserted, anything that comes after or at a later date has nothing to do with it.

Still il check a bit more.