The bug

1.21.1 pre-release 1 allows for customizing the overlaid texture displayed for some items. However, it does not work for the spyglass.

How to reproduce

1. /give @s spyglass[equippable={slot:"head",camera_overlay:"minecraft:misc/pumpkinblur"}]

2. Use it

Expected results

Using this spyglass would display the pumpkin blur overlay instead of the usual spyglass overlay.

Actual results

It does not.

Further notes

The pumpkin overlay correctly appears if this item is placed into the head slot, though.