Two defective C3909A toner cartridges in a row?

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I recently got a LaserJet 5si large format printer for dirt cheap ($50), with PostScript DIMM and 2000-sheet rolling tray.  It works, but has some print quality issues, so I decided to replace the toner cartridge to see if that helped.  According to the printer setup sheet, it has ~13k prints since its last service, and the manual says it needs servicing every 350k pages, so I'm hopeful there's nothing seriously wrong.  It's printed ~717k pages total.

It looks like HP either stopped manufacturing C3909A (renamed 09A some time ago) toner cartridges, or supply chain issues have impacted their stock.  I found a brand new one in a sealed (but damaged) box from a third-party seller, but it didn't work.  When you take these out of the bag they're sealed in, there's a bit of blue tape you need to remove, which is attached to a black plastic square that's partially inserted into the innards.  There's also a plastic ring you need to pull out.

Pulling on the ring, it pulls out another bit of filmy plastic from inside the cartridge, as well as a white aluminized strip as well.  After doing this, toner spills freely out of the slot they pull out of, getting absolutely everywhere.  Tipping it spills toner; squeezing it slightly makes it puff out.  This cannot possibly be right.

So I returned it and got a refund and bought a second, from a completely different third-party seller.  Exact same issue, toner just absolutely everywhere.

Am I doing something grievously wrong in my handling of the toner?  Is it possible that both cartridges were defective?  Are they old stock that goes bad in the box?

Any advice would be greatly appreciated.