Best thing to do is consultant with a tax accountant to see what’s going on.
But before you do that, contact the employer’s HR office and gather information asking them to explain your paycheck withholding saying “I believe there might be an error”.
Severance pay can be done differently. Some people get a lump-sum, such as the equivalent of 3 months of pay. Others don’t do that, they continue to run payroll as normal with an end day over those same 3 months. They do this in case they find out your somehow violated your severance agreement.
It could be just an error, where they treated your final paycheck without any regard to what you put on your W4. Or they put in the wrong numbers calculating it as if this lump-sum was what you normally get every pay period and that is how it got to 40%.
There are some places online, free I believe which help you calculate what to do with your withholding for a W4. Maybe Quicken or Turbotax web sites do this? I know I’ve seen it someplace online. Put in all the numbers and see if you can determine what they did.