Why troops living off the land lose readiness

Did Russian troops selling fuel in Belarus explain their early logistical problems?

That story is overstated; the deeper failure was an inability to move food and other supplies forward, forcing troops to forage and reducing their availability for combat.

Poor forward distribution left some Russian units without enough food and encouraged soldiers to seek supplies locally. An army that must forage cannot guarantee that its personnel will be present and ready when needed. The comparison is with the French fleet before the Battle of the Nile, when many sailors were ashore looking for food after Napoleon removed ship stores. Nelson's arrival found too many men away from their guns, turning a supply failure into a combat-readiness failure.