Need help solving a visual bug with custom Anvil Block

As it says above, I have a custom block similar to an anvil that can repair tools. My current issue is that when displaying the item preview, there is a visual bug that makes the preview flicker between the old durability and the repaired one. I have tried different iterations of making the preview be shown once and not every tick, but it either flickers or doesn't show a preview at all. If anyone has any advice it would great.