To my knowledge villagers were only to change profession if they remain in novice level too long. I have several Master level villagers who just randomly changed profession with no change to their claimed beds or Job blocks.
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Thank you for your report!
We're already tracking this issue at MCPE-62080, so I will forward resolve and link 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.
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Confirmed when playing on Bedrock Realm 1.14.1 on LG Q6 running 8.1 Android.
Leveled a cleric to master level just to watch him become a tool smith.