I was having this bug and saw a few others were as well. After a couple hours of troubleshooting, I've narrowed down the problem to do with conflicting playermodel within your mod list. I do not know for sure why these particular pms cause the issue; however, I've noticed a common pattern. Playermodels that cause this bug usually have custom animations, such as basic walking ones or ones that change the viewpoint themselves. The pm causing issue for me was the Among Us Drip pm, unfortunate ik. To find the pm issue in your modpack, just disable/reenable pages of mods until you narrow it down to one. Hope this helps.