Thanks for the extra information and project file!
The problem occurs because your reference scale is 0.01:
You probably want to set it to 100, which is the default. Do you know how it got set to 0.01?
Reference scale description:
Baseline scale factor for applying physics and other effects based on distance to non-scalable properties, such as angle or scale
角度やスケールなど、スケール出来ないプロパティに距離に基づく物理効果やその他の効果を適用するための基準スケール係数
When an entire skeleton is scaled, like during Import Data, many of the values are scaled to match. For example, if a skeleton is scaled to 10% of its size, then keys that translate a bone 100 will translate it 10 after the scaling.
However, some values are based on distance, but cannot be scaled. For example, physics constraint gravity is a force that can affect translation, scale, rotation, and/or shear. If we scale the gravity to 10% then it affects translation 10% less and everything looks the same (just smaller) because all the translation in the skeleton was scaled. That doesn't work for rotation (or scale) though!
Rotation is based on angles and is not affected by scaling the entire skeleton. Gravity affects rotation a certain amount at 100 reference scale. If you scale the skeleton to 10%, the way gravity affects rotation will not be the same. If you also change reference scale from 100 to 10 (ie 10%), then gravity will affect rotation the same as it did at the 100% skeleton size.
The exact value of the reference scale doesn't matter much. What matters is that you design your physics constraints to behave like you want at a given reference scale. If you then scale your whole skeleton and you want the physics to behave identically, just smaller or larger, then you also need to scale your reference scale by the same amount.
A reference scale of 0.01 is very small, making the physics properties extremely sensitive. We'll add a warning to the skeleton icon when it is set this way. If you didn't change it to 0.01 on purpose, then it would be good to know how it got changed, as it can be very confusing.
