When I played my modpack with Double Slabs, I had to hold the shift key for half a minute before it did crouch animation, but when i removed it, the crouch worked normally.
I will send you all the mods and config so you could recreate it, since I didn't find the problem in the logs, and since GitHub has file size limit you can download them from this google drive link: https://drive.google.com/file/d/1Ri79iyE2TPp0wq4C4Jx9aw8kJdEGQmPw/view?usp=sharing
sorry to bother and thanks in advance