Step to reproduce :

• spawn a villager close to a composter
• wait for him to take the job
• make trade with him until at leat one of his trade is locked
• "zombify" & cure the Farmer
• wait as long as you want, the farmer no longer reset his trades

I saw MCPE-63311 but I don't think it's related as to confirm "my" bug I tested it on an empty world so there existed no other work station/POI in the world