The bug
If you load one crossbow in the mainhand and press F to switch to the unloaded crossbow, both are not loaded, but the arrow however has disappeared but not completely as updating the hotbar causes the arrow to come back.
How to reproduce
Get 2 crossbows, put 1 in the offhand and the other in the mainhand
Get 1 arrow
Go to creative/survival
Load the crossbow in the mainhand then press F to quickly try to load the other
2 of your crossbows are not loaded, however the arrow disappears on the hotbar, causing a "ghost arrow"
Can confirm in 20w51a. This is quite difficult to reproduce and took me some time.