It took longer for me to notice, but this issue also happens when two separate Load stations are available at the same time. It was rarer to see as I didn't have many mines that were simultaneously ready for loading.

@Xyletic I agree, but I feel like it's a carry-over from the normal fixed train ...

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I encountered this while creating a Many to Many train network without using a mod like LTM or Project Cybersyn. Using train interrupts, I was able to do so, however, I noticed that some trains would stop at a Load station and go to idle.
I narrowed it down to train limits ...