Villagers i had before 1.16 still have the attribute of not giving many trades before selling out. This makes sense as to why this is and i am going to assume that that is controlled by a nbt tag that you dont want to change for all villagers automatically. but, if not, it would be helpful if our older villagers could be updated to the new amounts of trades.
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Thank you for your report!
We're actually already tracking this issue at MCPE-69757, so I will resolve and link this ticket as a duplicate.
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