The bug
Using a spyglass in the off-hand and then using a charged crossbow, the spyglass will stay in the character's eye.
How to reproduce
Put a spyglass on your off-hand
Load a crossbow and place it in slot 0
Now empty your mainhand and use the spyglass
Press button 1 to go to slot 0
❌ The spyglass stayed in the character's eye.
Can confirm. Relates to MC-203838. Just for clarification reasons, here are some clearer reproduction steps.
Steps to Reproduce:
Obtain a spyglass and a crossbow loaded with any projectile.
Hold the spyglass in your offhand and nothing in your main hand.
Switch into third-person view. (Hit F5 twice).
Use the spyglass and keep it expanded.
Switch from holding nothing in your main hand, to holding the loaded crossbow.
→ ❌ Notice how your offhand arm resets its position and is no longer supporting the spyglass.