Since 19w39a the players arm swings when he drops the item in his hand. If I remember correctly, this is an indented feature in the Bedrock Edition and thus probably is one in the Java Edition too. However: Me and a hand full of people I asked think that it is very annoying. For me it looks like a bug / unintended feature. This might not be the case for new players to the game, but it definitely is the case for me and many others.
Please overthink this feature and at the very least add a setting for it. I really don't want to be forced to play with a modded client in the future, just to disable this feature.