How does the federal withholding amount change on its own?

Hello! An employee brought it to our attention that the amount of federal withholding on their most recent paycheck was lower than it normally is. She hasn't made any changes to her W4 and nothing within the system has been changed, including her taxable gross amount that has been exactly the same for the past seven pay periods.

After doing some research, I found that this is the same situation on the majority of our salaried employees, approximately 1,000, for our most recent pay period.

If we haven't made any changes, what could cause this?

PUM 49 - PT 8.60.06

State - $U Federal - Tax table last updated 1/1/24