Hey! Have you fallen victim to the “bug” where when you import animations from blender that use an IK (inverse kinematics) constraint into roblox, the animation will play fine in the animation editor but when you get in game it looks awful? After countless hours and lots of bloodshot red and strained eyes, I finally discovered a solution to get your animations to play properly.
(In editor)
(In game)
It made no sense to me; why would my animations play perfectly fine in the animation editor, but not in game? Even baking the animations in blender which removed all presence of IK ended with the same result. Frankly, I was stumped. I spent so long working on this bug trying to fix it that my brain turned to mush and I was just throwing random stupid “solutions” at it, hoping it would fix it and WOULD YOU BELIEVE IT SOMETHING REAL SILLY FIXED IT. Let me show you how I fixed the IK issue so you don’t have to go through what I went through.
The first thing you’re going to want to do is go into the RootPart of your model
Right click on the RootPart and go to InsertObject>Bone
Name the bone whatever you want, I named mine controller because it’s controlling me from losing my temper from another failed solution, but the name really doesnt matter. Take everything under the rootpart that ISNT your new bone and drag them into the bone you just created.
Next, you’re going to want to go back into animation editor and export all your animations to roblox AFTER you’ve added the controller bone. If you try to use animations youve uploaded before adding the controller bone, it won’t work. The rig might look a little different, don’t worry. It won’t effect anything. (see how the controller bone is connecting the two IKs in the feet)
Update your animation IDs and load your character. If you did everything right, your character’s animations should play properly!
Let me know if this helps! *if you have issues with your animation controller itself then I can’t help you, this is just a solution for already working animation character controllers.
May you all be spared from what I went through. 
-supernob123