Operating system: Win 10
Graphics card: Intel integrated
Short description of error
When Mesh A and MeshB are union'ed, the merged result looks good. However, when a Mesh C is attempted to be subtracted from this union, there are certain conditions under which it does not subtract correctly. There is a good set of examples over here on Stack https://blender.stackexchange.com/questions/208861/boolean-modifier-doesnt-render-correctly/. Specifically when MeshA and MeshB are flush at the same level, there is a problem. However if you offset one of them by alteast 0.002 in Z axis, the boolean tool works fine. Even Fast/Exact options in 2.9 is not helping according to response from a contributor on Stack Overflow. Any help to resolve this without having to offset it would be appreciated.
Exact steps for others to reproduce the error
Based on the default startup or an attached .blend file (as simple as possible).