Ha, I found it.

I mixed up "join" in the formation menu, and "re-attach" in the unit menu. I did not use the join command, but the reattach command, and the difference and that there are two commans, escaped my attention because when translating it in my German mind I fail to see a big differen in ...

I tried just "load", and I tried "join" and then "load". Does not matter, no difference. They don't load up again.

Its my understanding that a line connects the squad with the transport it belongs to. If the line is yellow, I assume the troops should be in range to board, if its red, they are too far away and must be moved closer.

I think they should be and wish they would be able to be manually attached to a ...

If you buy at Matrix first, you later can download it again from Matrix AND Steam (after Steam release). If you buy via Steam, you have only one download source. Its probably just theoretical, but if one of the two sources falls silent, this might be relevant: you then have the alternative.

@ budd,

tried as you said, but it does not work. And I tried all combinations possible:
infantry close to its vehicle:
tried with yellow line visible, and tried without;
tried giving loading order instantly to both as you described;
tried reattach command only;
tried giving reattach command and ...

Did a test scenario to figure this out, found no solution.

Once dismounted, I find it impossible to get the infantry of the four Bradleys mount their vehicles again. I have them in the same square as their vehicles, manually order them there, I tried giving command to the infantry formation, to ...