I built myself a bunch of units (50), and had them hold position in rows outside a biter base.
After running in and gathering a bunch of biters I ran back into the middle of the crowd for help... and they did nothing.
Another biter even came and started gnawing on a unit that didn't react.
If I give a fresh hold position order, they all start shooting, but after a few seconds of no targets, they get lazy and stop fighting again.
I made some 4x6 concrete wall bunkers to stuff a bunch of units in with just stop orders, which is a little more effective; when an enemy shows up, they run around in circles banging against the wall like flies on glass but eventually some start shooting after a random amount of time (2-10 seconds). That's still way more than enough time for a solo small biter to take a few chunks out of the wall in front of 50 troops and cost me repair packs.
I'm not sure if it has anything to do with the biters being freshly spawned after the hold position order was given, or maybe if the units have a need to maneuver before firing even if the enemy is coming straight for them...
