From what I see of your setup, I think it's just because you're breaking and replacing the workstation. A villager will only lock a trade once you have completed a trade, otherwise everytime you replace their workstation their trades will change.
Did your totem of undying activate? I tried to replicate it but didn't see a problem. It might be that your totem of undying activated but you were still taking lava damage because it doesn't give you fire resistance and died afterwards.
Arachrys A
From what I see of your setup, I think it's just because you're breaking and replacing the workstation. A villager will only lock a trade once you have completed a trade, otherwise everytime you replace their workstation their trades will change.