Vivado automatically derives clocks on the output of a CMB based on the CMB settings and the characteristics of the incoming master clock. If the user defines a primary clock on the output of the CMB, Vivado does not auto-derive a clock on the same output. This DRC is reporting that a primary clock was created on the output of the CMB, which breaks the relationship with the incoming clock and prevents proper clock insertion delay computation. This is not recommended because it can lead to inaccurate timing analysis and incorrect hardware behavior.