I filled a drum with milk and left it sealed for a long time. I thought I would get kefir out of it, but the milk just got spoiled. So I cleared it with a plunger, but before it, I guess, I accidentally sealed it again. After it was emptied, it can’t be filled in any way and it can’t be unsealed.
Of course, it’s not critical at all, but I think sealing should prevent clearing with a plunger.
Also, I’ve did the same thing (fill, seal and clear) in creative and got the same result, so it’s repeatable.