When villagers interact with their job block, they are supposed to re-open their trades. This only works some of the time, and sometimes trades don't re-open for the whole minecraft day.
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duplicates 1
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Villagers interact with their workstations many time a day. They re-open closed trades only twice a day.
That's how it's supposed to work.
See MC-167002.
I found that too, and depending on your villager trading hall setup, they can sometimes try to pathfind to another workstation (even if they can't access it ( (at least they do in mine sometimes)). How i fixed it was to break the corresponding workstation(s) and place it down again