(single-player; survival mode; 1.16.3)

I am trying to nametag a zombie so that it won't despawn when i change the difficulty to peaceful.

I use the anvil, then place the nametag on the zombie. he shows the name one the tag.

But he despawns immediately when I change the difficulty level to peaceful.

I've reviewed the reports on similar problems. although playing in single player, I made sure i was connected to the server and waited a full day before trying to change the difficulty.

I've watched videos on Youtube of people successfully naming Mobs and there is nothing that I can see that I'm missing.