This is a known issue with Accumulator v11.0 and Accumulator v12.0
Core versions affected: v11.0, v12.0
Software affected: Vivado, ISE, System Generator for DSP
Simulators affected: All supported simulators
One workaround is to AND the Bypass and Clock Enable signals external to the fabric implementation to generate a new Bypass signal which will behave in the same way as the DSP48 implementation.
The resource and timing impact of this workaround is negligible.
Currently there is no workaround to make the DSP48 implementation Bypass behavior the same as the fabric implementation.