The function _calculateDeltaOfFSD of contract ABC incorrectly converts an int256 type parameter, _reserveDelta, to uint256 by explicit conversion, which in general results in an extremely large number when the provided parameter is negative. The extremely large number could cause a SafeMath operation sub at line 43 to revert, and thus the FSD tokens cannot be burned as _reserveDelta is negative when burning FSD tokens.
Simply calling fsd.burn after a successful fsd.mint will trigger this bug.
Recommend using the solidity function abs to get the _reserveDelta absolute value.
fairside-core (FairSide) confirmed:
