Keyword Model extensions are not supported without a Metadata Schema

When using dxa 2.2 (.net) and creating a custom Keyword Model _YourKeywordModelHere_ that extends KeywordModel, DXA fails mapping any field using that extended model, unless you specify a metadata schema for the keyword. 

Full details of the issue can be found here:
https://tridion.stackexchange.com/questions/20292/does-dxa-support-extended-keyword-models