Seemingly randomly (about a 60% rate of this happening) Minecraft will freeze (player can still move but any blocks destroyed will not drop their respective items and mobs cannot be damaged or interacted with; they also will not move. Basically an indefinite lag spike) when villagers level up from trades (particularly when most of a villagers EXP bar is filled by a single trade). I have only observed this in Hardcore single player.
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violine1101
violine1101
Reason for my resolution: This lag usually occurs when leveling up cartographers, since they have exploration maps in stock. When an exploration map is added to the offers list, the game freezes until it finds the target of the map. That's described in MC-126244.
If you're experiencing this issue with any villager professions other than cartographers, please leave a comment so that I can reopen this ticket and investigate further.
Thank you for your report!
We're actually already tracking this issue in MC-126244, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.