My guess is that relates to the Character IK component and the weights that you are using for that. Take a look at this video for an overview of the state system which uses the IK component as an example.
The State System is an incredibly powerful system built into the Ultimate Character Controller. Any game will have the character switching between component values as . . .
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The collider could be a red haring - that looks similar to what happens related to IK.
To clarify, I had the same issue and I went and changed the Hand Weight value from 1 to 0 in the CharacterIK component of the player. This is the same value listed in the MeleeCharacterIKPreset file.