Pack Leader provides Overguard when overhealing, and puts a max on Overguard that can be earned this way. (Reasonable.)
ISSUE:
This does not differentiate between Overguard sources (Pack Leader, vs Dante's Light Verse or Frost's Avalanche augment)
This cap will retroactively apply on a melee strike
If the mod is not equipped, the Overguard cap is 0 and will still retroactively apply
If a warframe gives a companion overguard from another source (eg Dante, Avalanche Frost), it will be applied as normal.
However as soon as the companion's owner makes a melee strike, Overguard will be lowered to Pack Leader's cap, or 0 if the mod is not equipped.
Results:
With maxed Primed Pack Leader, Dante gives their hound 23,000 Overguard, then silently destroys 20,800 of it on a single melee strike.
Without Pack Leader, Dante gives their hound 23,000 Overguard, then silently destroys all of it on a single melee strike.
Refusing to ever use melee, Dante gives their hound 23,000 Overguard. The Hound keeps the Overguard until it is removed by damage, as expected.