Issue Description:
When both PT and IBT rates are decreasing, _computeYield will enter the else statement where it will check if the expected and actual yields are more than the SAFETY_BOUND . The value checked against the invariant is converted to the decimals of the underlying token of IBT, but the wrong interface is used there. The underlying will be cast to the ERC4626 interface, but since ERC4626 inherits from ERC20, it will have decimals(). At all this only use the wrong interface, as the underlying is ERC20.
PrincipalTokenUtils.sol#L113
Recommendation:
Instead of IERC4626 , consider using IERC20Metadata
