Summary isn't an exact description of the bug, it's a bit more intricate than that and I'm unable to figure out what exactly is going on. Steps to reproduce:

1. Give yourself a zombie villager spawn egg (in your hotbar)
2. Place an anvil
3. Shift-click the spawn egg into the anvil, observe that the text position is correct (1.png)
4. Add a few letters to the name field
5. Shift-click the spawn egg out of the anvil input slot without doing the rename
6. Shift-click the spawn egg into the anvil input slot again
7. Observe the blue text highlight at the end of the text input slot indicating the wrong text position (2.png)

Note that this happens under various circumstances. I've seen inventory position play into it as well, and renaming the item wasn't always necessary. The reproduction steps above are the only ones that work consistently.