When all the slots of a hopper are filled and the hopper is locked by redstone, they will not accept items sent by droppers. I've built circuits using this mechanic before, so I know it's supposed to work.
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No it's not working as intended but it has already been reported as MCPE-22158.
I also have this problem, very annoying if you have a redstone creation where you need to use this. On minecraft java edition this works normally, I reported this too, but a mod said it works as intended, but it doesn't work as intended.