I've seen exactly the same thing several places:
"...small cells will attack larger units in hit and run in order to slow them down and to allow a "swarm" style attack when possible."
but in my observation that 'will attack' generally relies on one of two factors; either the COIN unit appears to be not too competent or unprepared at the time; or the insurgents have a really, really important to them area, item or person to protect. The former reason is the most common, the latter should usually be known if the Intel is fairly good.

Generally if a unit has its act together and looks like they really know what they're doing, the insurgents will avoid contact. I've seen that repeatedly even when they outnumbered the COIN force.

METT-TC, as always...