Could the previous owner have replaced the hammer with the wrong type for that gun?
A hammer which fits CZ 75 DA/SA and SA only pistols, when used with SA only guns, should not use the DA parts.
When a hammer is replaced, the safety, or sear should be fitted by someone competent. There are a variety of different hammer notch widths too. A hammer meant for Shadow 2 won’t work with decocker CZs, Omegas, P07, P09.
Hard to assess online. As much as you’d like to solve the issue yourselves, you should take the gun to a competent smith.
Best of luck solving this.