Preston randomly falls down and sits there with no available interaction for up to a minute at a time. Happens during dialogue and while he is just roaming, both while a companion and while he is on his own.
While at the same settlement I can watch him for a minute or two and will see him fall at least once. He stays on the ground like he is injured and waiting for a stimpak during combat but that is not the case, with nothing going on anywhere near him it still happens.
If it happens during dialogue the conversation is cut short, dialogue disappears and the conversation has to be restarted up to a minute later after he stands back up.
It was funny the first few times I saw it because the animation is pretty realistic like someone tripped over their own feet and slammed down on their ass. But when it interupts dialogue and combat and keeps occuring frequently it gets annoying quick.
Has been happening for near 20 hours now. Started happening around lvl 10-12, still happening at lvl 26.
None of the other companion NPCs have this problem.