I'm assuming yes but I'd like to get another opinion. A bag of holding's description states . . .
If a bag of holding is overloaded, or if sharp objects pierce it (from inside or outside), the bag immediately ruptures and is ruined, and all contents are lost forever.
Simple enough though it doesn't explain how shoving a dagger in there might pierce it but that's GM choice. The handy haversack say's . . .
It has two side pouches, each of which appears large enough to hold about a quart of material. In fact, each is like a bag of holding and can actually hold material of as much as 2 cubic feet in volume or 20 pounds in weight. The large central portion of the pack can contain up to 8 cubic feet or 80 pounds of material. Even when so filled, the backpack always weighs only 5 pounds.
So they're like a bag of holding does that mean they have the same piercing vulnerability or does it just mean they can store more than they appear to be able to? Similarly I assume the large central portion should also be included in this description. So my question is am I right that a handy haversack being pierced loses everything?
On a matter of curiosity why does overloading cause them to explode rather than just spill out the top?
It isn't crystal clear, but we've always played it as having all of the same properties save for the differences mentioned in the description. Since it's a haversack, it might be a bit sturdier than a "common cloth sack", but we're back to GM discretion there.
If a handy haversack is pierced, though, it should only cause the loss of the things in the pockets pierced. The other pockets ought to be fine, as the haversack is basically three smaller bags of holding stitched together. (Offer void [pun intended] if portable holes get involved.)
Notice that being an enchanted item doesn't make the handy haversack or the bag of holding more resistant* to piercing that the normal versions of those items. So a burlap or leather bag or haversack has only a few hp and low hardness (but generally they are immune to blunt damage).
From inside, generally, the bag or haversack will take damage only if there is a creature within it and he/it attacks the inside of the bag, from outside they can be an easy target of sunder attacks.
*It is possible to argue that the way in wick they are enchanted makes them more susceptible to piercing damage, as a normal sack or haversack will suffer little damage from an arrow piercing it. A simple hole that can be easily mended. The magic version will ropture and lose its magic.