When I write a category containing a comma, it works as a splitter and other categories are created. For example, "Compilers, Interpreters" are splitted and create a pair of new categories: "Compilers" and "Interpreters". So, "Compilers, Interpreters", "Compilers" and "Interpreters" form part of categories. Reproducible: Always I'm using Kontact, version 4.14.4 (I don't found it on "Version" menu here).
For me it's fixed in last version