When the right hand does anything, the left hand bobs as well. This did not happen before the 1.16.210 update.
Steps to reproduce
1. Go into any world in Creative Mode.
2. Grab a shield from the Creative menu and put it in your offhand.
3. Grab some blocks and tools and switch to survival mode.
4. Start breaking blocks, placing blocks, switching items, using tools, using items, and picking up blocks.
The shield does not bob whenever a tool is used, blocks are places, an item is switched, an item is used, or a block is picked up.
The shield bobs whenever a tool is used, blocks are placed, an item is switched, an item is used, or blocks are picked up.
The shield did not bob when a tool was used a block was placed, an item was switched, items were picked up, or an item was used.