The bug

The attack indicator fills at a completely different rate than the item raises.

1. By the time the item enters the screen, the attack indicator is more than halfway full:

2. By the time the hotbar indicator is full, the item is still mostly off-screen. It continues raising for a moment while the indicator remains full:

Hotbar Indicator:
Item begins raising at 50%.
Item stops raising at ~125%.

Crosshair Indicator:
Item begins raising at 50%.
Item stops raising at 100%.

How to reproduce

The animation is very quick. It's noticeable with any item, even non-weapons. To see the discrepancy precisely, use a weapon with very slow attack speed:

/give @p diamond_sword{AttributeModifiers:[{AttributeName:"generic.attack_speed",Name:"Attack Speed",Amount:-3.8d,Operation:0,UUID:[I;9,7,5,3],Slot:"mainhand"}]}