The spotting of an enemy or a firefight cause the AI team to split itself in 2 files for a time of about 280/300 seconds and the behaviour should be related.
The attached mission test:
After that AI group has killed the opfor man when the area is clear you'll notice that they walk in 2 files instead of a staggered column, to avoid this bug in the second waypoint I've inserted 300 seconds under waypoint timeout min mid and max, in fact I've discovered that this stop resets the AI formation(even behaviour?), otherwise you'll see the column moving in a strange way.
I hope this could help you to fix it soon
{F19663}