Version 1.2.0 is not backwards compatible with 1.1.x. For example, the constructor of OpenApiString has changed from:

public OpenApiString(string value);

to

public OpenApiString(string value, bool isExplicit = false);

If another project references the old version, it cannot find the constructor since its signature has changed. It would have been better to add a new constructor or increase the major version number of the package (if you adhere to semantic versioning). At the very least, breaking api changes should be documented in the release notes.