It has happened to me twice. The first time I was breeding villagers in order to repopulate a dead village. The second time was in my "trading hall" with the villagers with the best trades I could find. In both cases several of them disappeared, even though the rooms were well lit and the villagers were completely locked, so it wasn't because of zombies. I tried putting tags to them, but they keep disappearing.
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We're tracking this issue in MCPE-21416, so this ticket is being resolved and linked as a duplicate.
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Duplicate of MCPE-21416