Whenever they are commanded as a groupd to move they decide to use the current formation as their destination formation. This means that every time any slight positional change affects them those differences are saved in their new destination and the formation drifts apart randomly. This means frequent commands (such as follow mode) and/or different move speeds will quickly alter the formation enough that I need to take frequent breaks and reposition them into formation so that they say within range of the ones with roboports that repair.
When they have different move speeds the drift is so extreme that almost any time you order a new position before they arrive at the previous one the formation gets completely broken and they might drift several chunks per click.
Suggested fix: Save formation when they are selected instead of when they are ordered to move. This way you can reorder as many times as you want with completely different amounts of exoskeletons (or battery charge difference) and no drift will occur.
Edit: Bonus bug: If you enter a spidertron and use follow mode, your spidertron will try to follow the spidertrons that are following you and keep danceing back and forth. This happens regardless of the fact that your spidertron is part of the group that is following you or not. But it should never try to follow you when you are in it...